Review writing is in fact a type of journalism, and is generally classified as opinion writing. As such there's few hard and fast rules about how to go about it. Success as a reviewer mainly depends on your appeal to your audience, whether it's the quality / insight of your analysis, your sense of humor, or the beauty of your writing that draws people to you.
Facts are important in all forms of journalism, including reviewing, as they are the backbone of readership trust and your credibility. However, it's not facts that people generally read reviews for. They want personality and analysis, new ideas, in other words, to contextualize the facts.
One thing you could do is look at different approaches to reviewing. Since you're already looking at examples you've probably already noticed a variety of approaches: satirical reviews, scathing rants, light fluff pieces, deep analytical takes, provocative political tracts, and so on. There's also video reviews, which typically rely on written scripts, so don't discount those either. Try to cast a wide net to see everything reviewing can offer, then drill down into the specific variant you want to master and try to develop a style.
So I'm afraid there's no real "foundation" to reviewing beyond the very basic tenets of journalistic integrity and doing as you already are doing in reading examples in the wild. If you want to brush up on your journalism ethics, there's plenty of places to do so. As an opinion writer you're free of the need to refrain from bias in your writing, but you have just as much obligation as reporters to not mislead the public by publishing false information. Always point out when something is your opinion and when it's a fact, and if it's a fact, where it came from.