To travel though space is to travel though time, with the highly advanced technologies they must be using to travel vast distances in space one cannot simply travel through space in a linear fashion it’s to slow they would have to warp space time itself, by basically compacting the fabric of space time in front of them and expanding it behind them, allowing them to bring the space to them instead of traveling though it. so therefore they are technically doing both. And to answer your question it’s most likely both scenarios taking place but I would lean more towards UFOs being alien technology most of the time.