Now that is absurd! - Nobody has ever offered a shred of proof that the "UFOs" traveled through space to get here, thus from whence comes the assumption on your part that they are "space ships?"
I think you’re mostly correct on that score.
Inter dimensional is also quite plausible.
That’s kind of been the whole topic of discussion for the last two days. Some folks saying aliens are really fallen angels and that their crashed or possibly not crashed vessels (”space ships” to keep it short) are the source of most of the technology that made the thing you’re sitting in front of possible.