Sideloading is a term that's been around for decades, it's not some made up word by tech giants to make people scared of installing apps.

The term originates from a designation for transferring data between physical devices and was slowly adopted (because language is fluid) to its current definition (by people on forums like xda).

This isn't some conspiracy and Google and apple don't need to use coded language to prevent you from side loading, apple for example just outwardly and bluntly forbids it.