Anyone who believes in these alien UFOs simply does not understand anything about the immensity of space, and the virtual impossibility of travel between star-systems.
But how can we test this idea? Here’s how, fellow earthlings! Consider that organic beings would find (just as we are learning) that it is much more feasible to send out robot devices (or drones) than to try to move organisms over vast distances. So, in all likelihood, if there were space aliens, they would send out exploratory probes, unstaffed by any actual individuals of their “people.
Now, we have fossils from hundreds of millions of years of Earth history, and so far we have found absolutely no mechanical devices predating humans. Nothing.
It is really simple. If there are other intelligent beings somewhere else in the Universe, they are impossibly far away. They can’t get here from there.
Go read some of the forbidden archeology genre. There has been stuff found hat appears to me mechanical or electrical embedded on rock strata where it shouldn’t be. And because they were found where they weren’t expected...they were dismissed as unexplained.
I agree that the vast distances make space travel improbable. I think a vast majority of UFO activity is earth based military stuff. But I would not dismiss something dimensional or something along those lines.