I believe they exist, and I believe they’ve visited this planet. I don’t think all UFOs are “alien” or extraterrestrial (such as plasma UFOs); they may be some sort of yet undiscovered life form living on this planet, which may be very alien in itself.
I think some “craft” UFOs, like saucers, could be some kind of robotic probe sent out by aliens. not necessarily spacemen piloting spaceships.
there’s also the assumption that these objects/beings are coming from a distant planet, when it should be considered that they may have some sort of residence (colony?) on or around our planet. Perhaps they’re not even extraterrestrial, but exist in some parallel universe and their craft are some sort of inter-dimensional craft, rather than interstellar.
But there’s really no way of knowing, unless they were to reveal themselves formally to us. They sure don’t seem to act very human-like from their alleged interactions with people