Personally, I prefer using the MDN docs [0][1] for web extensions as I am using mozilla's polyfill library [2] anyway. These contain more practical examples and less abstract descriptions at times, including a very thorough getting started guide [3] going over everything including various concepts.
The point remains the same though, the basics aren't all that difficult and while Plasmo does seem to make some aspects easier it also seems like overkill in other areas in regard to abstraction.