You can read about all the changes in 4.0 at the release notes: https://wiki.blender.org/wiki/Reference/Release_Notes/4.0 . Usually, there’s also a page on the blender site around the release, highlighting most of the changes (example for 3.6).
As for understanding, the release notes (and links within) are a good starting point, but might be a bit too technical at times, and require some familiarity with many aspects of Bender. You might want to check youtube for that, altough the content will be scattered in various videos, and they rarely go in depth or over smaller changes. Pablo’s livestreams are probably the ones with most content about the changes.
There’s also many threads here in BA (for example, this one for color spaces/filmic/AgX, other for geometry nodes ).
But I don’t expect many people to go over all the changes and fully understand them. It’s a lot, it’s probably better to just use it and research as you go.