I think that we can only look at this possibility of how they travel through space from our own abilities to travel through space, and other extraterrestrial civilizations might have something totally different that they use to travel because they are so far advanced from where we are at right now.
A person can get on an airplane in New York City and be on the ground in Portland Oregon in a matter of a few hours.
However, it took the Lewis and Clark expedition months to get out there, simply because their ability to travel was so far less advanced than ours is nowadays.
If these extraterrestrials look similar to us, or have the capability to do that, then they could have been here for centuries, and we would never have realized it.
I find it interesting that we have civilizations that live in remote places, like in the Amazon jungles, that have never developed beyond a very primitive culture, and at the same time, we have other countries where progress is being made all of the time.
It makes me wonder if maybe we have had some help and information that took some of the people on this earth further in development than we would have done, if left on our own.
Would we still be living in jungles or caves and hunting our food with wooden clubs and throwing rocks ?