As far as UFOs, Occam's Razor.
Interstellar travel is extremely difficult and one way, forget Warp Drives, etc., the only "faster than light" that is feasible might be information through some sort of Quantum effect - but these effects aren't sustainable at macro (i.e. real world) levels. So slower than light propulsion means century long trips.
In that case, odds are the only aliens that come here are coming to stay, and will see us as inconvenient at best, and food at worst. That is, they would come in "generation" ships on a one way trip, looking for prime real estate. Which means a carbon based life form, silicon based life forms might not consider Earth to be attractive. Though a gas giant evolved life form might prefer Jupiter.
What might come in small numbers would be AI guided robot probes, b/c traveling a few hundred years wouldn't be a problem. But we've only been broadcasting our presences for a 100 years or so, which makes it unlikely that anyone would have launched a probe toward us before then.