I may be the only person who ever understood every detail of C++, starting with the preprocessor. I can make that claim because I'm the only person who ever implemented all of it. (You cannot really know a language until you've implemented it.) I gave up on that in the 2000's. Modern C++ is simply terrifying in its complexity.
(I'm not including the C++ Standard Library, as I didn't implement it.)
Sean Baxter single-handedly implemented all of up to C++23, and some C++26, including a huge number of GNU extensions and possibly an even larger number of his own features.
I don't know much of anything about him. Did he implement the preprocessor? the optimizer? the code generator?
(For some context, back in the 80's, code generators needed enhancements to implement C++. You couldn't just use an existing one. Bjarne had to do some ugly workarounds because of this.)
Sean Baxter's circle compiler uses LLVM as a backend, but I believe the rest is from scratch.
Arguably these days having a clear frontend/backend separation is good compiler architecture. It might slow down compile times a bit, but it's worth the cost.
It wouldn't have made much sense to write the preprocessor these days, too, but it is part of the C++ compiler. Unless integrating it with the C++ lexer for speed purposes, as I did.
P.S. we're adding an "Editions" feature to D so we can simplify the language by removing obsolete and deadend features. We didn't get everything right, and want to fix it!
(I'm not including the C++ Standard Library, as I didn't implement it.)