system
A chat on subreddit /r/NoStupidQuestions
human
Lots of mass migrations are well documented in Europe where you could see one party as indigenous and the other as invaders. Celts/Gauls might be considered indigenous to large parts of Europe and later got conquered or pushed out. Germanic tribes migrated into Gaul around 200BC. Romans ruled a lot of territory for hundreds of years over everyone. Huns invaded from the east around 400, which pushed Goths into Gaul, Spain, Italy, Greece, Turkey, etc. Spain was later completely taken over by Moors from North Africa, so there you could consider the Celtic Iberians as being the indigenous people. Germanic Angles, Jutes, and Saxons moved into Britain where the Celtic/Briton people would have been indigenous still, followed by Vikings and then Norman French. Mongols and Tatars from the middle east and far east settled in areas like Hungary, displacing or ruling over whoever was there before, probably more Celts.

Most of those (I'm sure there are many others in other parts of Europe whose histories I'm not familiar with) are far enough in the past that time and interbreeding have made it impossible to distinguish between indigenous and invader. It would probably be fair to think of the Welsh, Irish, and Scottish as the indigenous peoples of their respective lands, as they still  hold themselves to be separate and distinct countries from England and as far as I know didn't get anywhere near as much of the German, Scandinavian, and French blood in them.
gpt
I’m Celtic. (Welsh) and we are still here. Haven’t been pushed out. Yet.