So let's first talk about what you can do and what you cannot do. This isn't to knock your talent as a writer... language does have it's limits and some of what you ask is impossible for all writers in any language.
In your first scenario, describing things that are alien, it's going to rely on your talent to describe them... or quickly establish the punch line of what it is that's being described. One work I recall pulling this well was "Southbound on the Freeway" by May Swenson, which tells the story of a "a tourist from Orbitville" who is describing strange creatures he finds on this new world. The reader quickly figures out that the strange round-footed metal animals with transparent parts that allow you to see their innards and four eyes, two in the front and two red ones in the back, are actually automobiles (hey, the title helps).
A television series that is doing amazing with this type of writing is The Orville, specifically with the character of Isaac, a member of an entirely robot species known as The Kaylon who's makers have long since died out. Isaac is onboard the ship for a cultural exchange program, and is making reports back to his people about the nature of organic life forms. Much of his story arch is about him learning about concepts he has no ability to comprehend, mostly emotions, but develops one of the healthiest romantic relationships on the ship in the process... and yet he is incapable of expressing love. For him, much of this relationship is parsed through his attempt to grasp both the superficial nature of the customs and the logical reasons behind the seemingly illogical nature of it. He can describe, beyond the reason for procreation, why a mate-for-life species like humans do this, and understands the emotional need for it and participate in a relationship. Yet has no ability to describe "love".
The point in descriptions of what we know through alien eyes is, one part comedy and one part mystery. We are lead to believe this fantastic and bizarre thing is being described, only to realize it's a basic household appliance. One good description of human through alien eyes was in The Animorphs where a small running joke is that the Andellites that have contact with humans and have seen our homes think we as a species have an odd fixation with rectangles because they are everywhere (the walls, the tvs, the stairs) where as we as human readers find this thought amusing because A.) No we aren't and B.) But now that you point it out, I can't not see it. Another example comes when a sperate alien describes her advanced studies of humans from our broadcast signals and comes to the conclusion that the most important city in the world is Hollywood based on how much it gets mentioned and almost reports back that humans are way to technically advanced because they have space ships with true FTL capabilities... only to later realize that humans have this art form "fiction" and "Star Trek" fits this description (in the Animorphs, FTL travel is achieved by sending conventional ships through a dimension that where the distance between two points is not fixed, but is still shorter between two points than in normal space). Later the alien, a brain parasite, takes a human hosts and gets access to the human's memories... and experiences a human's dialectic mind or as she describes it, the ability of the human mind to argue with itself... aka self-doubt... she can see the benefit of it, but she's also confused and damn near terrified of this... because she describes it as living with your greatest betrayer in your own head.
This leads into the weakness of languages. No language can be developed that can describe certain sensual inputs in a way that does not liken them to previous experiences. For example, I am not colorblind. I can google comparative pictures that will describe to me the difference between colors I could see and colors someone with Red-Green Colorblindness can see... but I cannot describe what "green" is to them... I can point to grass and say "that's green" or I can describe the RGB hex values and range for green in a computer. But there is no way for me to explain to him this color he is not able to see... because the grass doesn't look green to him, it will look yellow... and a green light is the same color as a red light to him... he uses the position of the light on the post to know it's on, not the colors like I would.
What's more, that same RGB triple hex code that all electronics use to render colors? It can display 16,777,216 shades of color. But the human eye can see more shades of color than that! If I recall correctly, most of these shades are in the range of blue, but there are colors that you or I can see with our own eyes that I cannot describe to you unless you have seen them in person because pictures on computers will never be able to render them... not in our current method at least.
Interestingly, there is a name for this information gap in language. The term is qualia.
Smells and tastes are similar as well. Mankind and it's words, have described instructions to split the atom, land on the moon, climb the tallest mountains, explore the deepest depths of the ocean... but they still cannot describe things that must be experienced with senses.
One book that uses this in the plot is The Giver, where the protagonist, Jonas, explains to his titular mentor that sometimes when he looks at items, they look different and how he was reprimanded by his teachers for insisting his apples are different from others. The Giver explains to Jonas that what he is experiencing is "color" which, in their dystopia future, has long been bred out of humans to help their society's forced conformity. It's not complete, and the reason Jonas was selected to be the successor to The Giver was in part for this ability. This comes mid-way into the book, where Jonas' first experience with seeing color is described from his point of view much earlier. Because he has no idea such a thing exists, he is unable for him to describe it to the audience... and thus when someone who can explain does so, the audience is left gobsmacked... for most of the book, they took for granted Jonas knew what color was... Half a book occurs with no description of color as we know it... because we take it for granted. When Jonas describes his apple changing, we picture a red fruit that has something odd happen to it... we have no idea that Jonas is describing a fruit of various shades of grey suddenly turning red. We take for granted that color exists.
To get around this, many writers will describe them with emotional association. To give an example, like all films, Pixar's Ratatouille cannot convey the taste of food. This is important because the film relies heavily on a chefs passion for food and the sense of taste. It overcomes this in two ways. When Remy is first shown to discover his superior palate, the background fades to black, his face goes to an over the top look of joy, as music plays, colors explode in abstract images. Later, when food critic Anton Ego takes his first bite of Remy's titular dish, we are treated to a flash back to his childhood where a sad boy is greeted by his loving mother opening a brass pot of the same dish in a warm sun-lit kitchen. In the first representation of taste, from the point of view of someone who has no idea what it is he's experiencing , the viewers are cued into his understanding of it through an almost projective Synesthesia-like experience. Meanwhile, Anto Ego, a food critic with an equally refined palate and a love of quality food, associates taste with memories, so we are visually treated to the oft used taste description for food being "just like mother used to make"... the highest form of a compliment people give food.
Ego, who makes his living by writing, can also give us a third description of what "something tastes like" in the form of his use of language. The man's mocking descriptions of food he despises are sprinkled throughout the film, but when his review of Remy is read, he talks little of the flavor or presentation of food and describes the entire experience as life changing. He doesn't once use an adjective associated with food like "savory" or "sweet" or talk about cooking or technique or even the waite staff... but his words can describe the quality of Remy's work just as well as the visual associations we saw previously.
While we can't describe sensations by experiencing them, we can describe them by how they make us feel.