Interdimensional aspect could be Earth from an alternate timeline, such as where the Cretaceous–Paleogene (K–Pg) extinction event did not occur, and reptilians not mammals evolved into an upright "humanoid" species that developed civilization millions of years ago. These interdimensional reptilian humanoids (from an Earth literally "overlaid" on our world) could be in a war with our timeline/dimension for some interdimensionally-scarce resource, and perhaps they are what religions call "demons" and perhaps their version of Earth is what is known as "hell". Perhaps, sometimes when someone sees an "alien" or a "UFO" or a "ghost" they are just lucking into a naturally occurring transient random portal between dimensions, and when those things suddenly "vanish" it just means that portal closed/the conditions that gave rise to it subsided.

It could also be refugee species, advanced civilizations that came to Earth millions to hundreds of millions of years ago (perhaps multiple races/waves over many millions of years) and temporarily resided here after some cataclysm or disaster beset their civilization, before moving on. Some stragglers may maintain outposts on Earth for whatever reason.