@protoseepp, there is currently only one compiler setup that is fully 100% C++20 compliant. Visual Studio on Windows. All other compilers lack key features, arguably the biggest being modules. <format> is another MIA feature.
No compiler is 100% compliant with C++23 yet, officially documented.
I can recommend several books/eBooks I've found useful, including a bundle of 3 eBooks that cover in depth key ideas of programming.
A print book for learning C++, C++20 at that though it still covers "older C++", is "Beginning C++20: From Novice to Professional".
The book covers quite a lot of material quickly, building on previous examples.
It is not your typical "let's teach C++ as if it were an enhanced C" approach far too many imperfect/bad C++ books and classroom courses follow.
If you get the book and are able to use Visual Studio I can give some suggestions to help make using modules less of a PITA as they were for me at the start.
|Oh no. I have not even picked up my C++20 book yet.|
That is an ongoing issue with the evolution of C++, new things to learn and deal with. Even the guys who write the books have to learn the new features as they are introduced before they can show others.
Luckily C++23 doesn't appear to be as sweeping a change as C++11 or C++20.
Others here are C++ experts, AFAIK actually C++ working professionals. Me? I'm a self-taught programming hobbyist. Being solo learning since before C++98 was the standard.
I'm a crotchety old fart with some very strong opinions about things C++. :Þ