In my story, there is a spot where there is a major shift in perspective as one point of view is supposed to be from someone writing in the first person to another point of view in the third person. Is it better if I explain why the person writing wrote it like a story even though its a diary or is it okay to assume that the audience would understand why? Should I subtly give an explanation, make it obvious, or not give one at all?
You can write the story any way you like, but you run the risk of breaking your reader's attention to your tale, by suddenly making changes to your method of telling. Instead about thinking about the welfare of your characters and the progress of your plot, they might start thinking, "Why the change in POV?".
To minimize that distraction in this case, why not have your third party narrator announce that the following comes from the filed report of the young detective; before reading the report (in first person) to the reader.
This unifies the whole book under a single point of view, while still allowing you to control information and insight by using first person for the first part.
In The Sound and the Fury, Faulkner tells each part of the novel in the voice of a different character. In No Country for Old Men Cormac McCarthy switches back and forth between first and third person narration. In Bleak House, Dickens switches back and forth between a rather haughty and detached narrator and the very warm and sympathetic voice of his heroine Esther Summerson. In short, it can be done.
At the same time it is an advanced literary technique, and, perhaps more importantly, it is a self conscious literary technique. It draws the reader's attention to the fact that they are reading a literary work. This is not necessarily a bad thing. The sophisticated reader is quite capable of engaging with a story on multiple levels simultaneously. Done well, the technique gives an additional layer of interest to the story.
But by calling attention to the literary technique of the novel, this approach goes against the the cinematic approach to the popular novel that is prevalent today. In this approach, the author tries to create an experience for the reader that is as much as possible like watching a conventional movie. (Movies, of course, can use self conscious cinematic techniques as well, though it is not common in mainstream cinema.) This is where the "show don't tell" doctrine is applied in full force. (It is a literary technique, not a universal rule.) The aim of this technique is that the reader/viewer should forget that they are reading/watching and should just experience events as if they were there.
When you use explicit literary techniques such as switching POVs you shake the reader by the scruff of the neck and say, "Hey, pay attention, this is a book you are reading." There is a very long tradition of such books, so it is perfectly valid to do so. But know what you are doing because it changes your audience.