Yeah, it's all done automatically when you build, and dependencies are properly taken into account: if you modify one of the code generating sources, its outputs are regenerated, and everything that depends on them is correctly recompiled. This doesn't take much CMake logic at all to make work.

In my case, no, it's dumb old code writing strings and dumping that to files. You could do whatever you want in there, it's just a program that writes source files.

I do use some template metaprogramming where it's practical versus code generation, and Boost Hana provides some algorithmic facilities at compile time but those incur some runtime cost. For instance, you can write a while loop with bounds evaluated at compile time, that lets you use its index as a template parameter or evaluate constexpr functions on. But sometimes the best solution has been (for me, performance/complexity wise) to just write dumb files that hardcode things for different cases.