I only tweak my acad.lsp & acaddoc.lsp when newer system variables are added or when something new like command-s starts working better than command but there are other ways to load code like adding *.mnl (menu lisp) files with the same name as a custom *.cuix file. Most installed *.cuix files already have an associated *.mnl file. While I wouldn't recommend modifying any installed files they are worth exploring as you may find the functions they add useful. c3d.mnl added for Civil 3D users adds 11 zoom shortcuts as well as alerts and menu functions included with most *.mnl files. For loading *arx files you can add an *.arx file with the same name as a custom *.cuix file. I used this for a few versions many years ago.

Inside the cond conditionals described earlier many call separate lisp routines to load or run functions though I prefer not to.

Use autoload (AutoLISP) to load functions you run by typing at the command.

Loading order of AutoCAD startup files: https://cadpanacea.com/wp/?p=671
Modifying installed files like acad2021doc.lsp is neither recommended or necessary. Even done correctly they could be replaced with an installed update.