Also, they seem to be written primarily for the "mupen64plus" core - which as of the most recent time that I updated my pi4 is no longer available, having been replaced by the "lr-mupen64plus" and "lr-mupen64plus-next" cores.
lr-mupen64plus is the mupen64pus core (now superseded by lr-mupen64plus-next, I think you're confusing it with standalone emulator (which is named mupen64plus).
Multiple google searches have actually yielded a truly distressing lack of documentation. I haven't found anything remotely resembling a set of useful instructions. Does anyone know how you're supposed to get Hi-Res Texture Packs working now?
Standalone instructions https://github.com/mupen64plus/mupen64plus-video-rice or the docs page, libretro core(s) https://docs.libretro.com/library/mupen64plus/#hires-textures. The N64 opt page (here) has even a video linked with instructions for standalone Mupen64Plus (not the libretro core).
I agree that documentation could be more explicit, but should get you started. Maybe give more details on what have you tried so far and it's not working?