Nah, they're not actually aliens. They just take forms that aren't really recognizable as any known species. They're basically the 'other' classification for Kaiju, where any Kaiju with an unrecognizable species gets put. Xenoforms, not Xenomorphs, usually look alien or chimeric, often have insectoid qualities, tentacles, or other traits that you don't often see in Legends, and Xenoforms are pretty much all radically different from one another. With legends, you have sort of 'subspecies' like dragons, Tikbalang, and Kelpie, but every Xenoform might as well be it's own subspecies. Xeno more refers to 'strange' in this context, rather than 'alien'.

And no, we don't have interstellar travel. That's a bit beyond our capabilities at the moment.