dnd 5e – What are the DnD Sourcebooks (or simply the sources in general) considered as official for game purposes compared to what is not?

I know that there is something similar to this @ nitsua60 question, but I want to ask something a little different. This question was about what is official, but I would like to ask questions about the elements that confuse officials in DnD 5e, because they sound good or still play test material. I want to ask this question because I'm introducing someone to the game, and even I'm a little confused about some of the sources I'm looking at online for myself (like Wayfinder's Guide that Eberron is on DnD beyond, but apparently still playing test material?), So I want to better explain some reasons why the sources are technically unofficial, instead of what they are.

So, basically, I would like to ask what are the main reasons why source books (or simply sources in general) are often confused with being official (or usable in the official game), but do not are not really, and what are some common examples. Thank you and I hope this can help more people than myself.