It’s very easy to pick up. Out of the box, it’s just Evil, Ivy/Vertico, Org-mode, and several programming modes. The spacebar is likewise employed for many actions, but I don’t use most of them myself: just have about a dozen that I invoke regularly. The enabled modules (readymade configuration) and installed packages are specified in config files, and doom sync handles installing them.

It has some emacslisp helper functions/macros to add mappings, add hooks on modes, etc. — these are more convenient than those of raw Emacs.

I’m not sure why the author switched Doom to Vertico in the upcoming version 3, when Ivy was working fine. I’ve made some configuration tailored to Ivy, so enabled it back via the config file.