Yeah most call it homebrew, and you can package them up as wads or use forwarders rather than homebrew channel or whatever.
For homebrew then while some official code has been leaked you would be better off with something like
https://devkitpro.org/ with a bunch more libraries and other such things on
https://wiibrew.org/wiki/List_of_development_tools
https://wiibrew.org/wiki/Main_Page (one of the better options for hardware documentation, though dolphin emulator source code is good stuff) does note a few higher level languages being made available for the wii over the years but not so many would want to use them in anger compared to some other consoles where there was a bit more of a flourishing scene.
https://wiibrew.org/wiki/Lua_for_Wii https://wiibrew.org/wiki/WiiPy being the main two. Lua and Python both being far easier languages to get to grips with than the usual C family stuff that most use on stuff like this.