I wrtote a reply to this question in a comment parallel to yours.
In addition:
I don't write many classes in the sense of the Gang of Four book. I do write classes that are really just ways to speak with the type system, e.g. a "class" that has one instance variable that is an integer, so takes up only as much space as a native int, yet perhaps acts a special kind of table index.
Sequence manipulation, auto, and destructuring bind provide clearer ways to express algorithms without getting hung up in towers of class abstraction (an evil fetish) yet not lots of low-level manipulation and boilerplate like you'd see with C.
In c++20 generics and templating are almost unified which makes expression so much clearer (most of the old templating syntax is unnecessary in most cases). It lets you manipulate the type system much more clearly. You can do a lot more at compiler time -- you'll never get the expressive power of the Lisp macro system, but it's a lot closer with a lot of the old syntactic junk discarded.
Auto declaration has transformed the use of the type system and genericity.
In terms of resources to learn this stuff: I haven't found any single good source of info for these revisions.
I once wrote a comment on this but it took a while and of course vanished into the whirlwind of YC comments. So instead I'll suggest a web search for "new in c++17" and "new in c++20". I know that's kind of lame, but that's the best high level answer I can suggest.
I will say www.cppreference.com is excellent, but when it comes to newer features you often need something a little higher level to really grok what it says. It might answer your specific question though.
I follow the trip reports from committee meetings and read proposals that happen to interest me which often give context to a design decision.
I started a new project (blank editor buffer) using C++-17 in 2016. I hadn't written c++ since around 2000 (though I'd previously been involved in g++ development since around '87.* So I treated it as if it were a brand new language I'd never seen before. The only book I bought was Stroustroup's "tour of C++" which introduced c++14, then used searches like the ones above to keep up with developments in C++.
If you're in the same boat I'd say choose C++20, even though the full language isn't available yet, which is what I did with C++17. Some features I was able to bring in from Boost (which is where many new features begin) and some I just couldn't use until it was time to refactor something anyway. Still, this really made a difference.
* yes before g++ was released by the FSF -- tiemann and I both worked at MCC at the time and used to have dinner at 3AM to discuss what each of us was working on, which is how we later came to start Cygnus.