Companion to the bans piece but focused on the scenes themselves rather than the ban question.

Singleplayer modding

Atmosphere modifications. Custom interiors. Story-additions. Texture overlays. Vehicle replacements. Largely a hobbyist scene.

Tolerated by Rockstar. Slow-growing audience. The kind of mods reviewers cover.

Online / multiplayer modding

Custom server frameworks (FiveM, RAGE MP). RP scenes. Custom gamemodes. Large player base. Often commercial - servers monetise.

Tolerated by Rockstar in framework form (FiveM acquired). Aggressively prosecuted in cheating form.