This primer from the Washington Post gives a reasonable breakdown of the ethnic situation in Spain, and who ETA are, but it makes a curious mistake
The Basque people, believed by scholars to be of Celtic origin, have inhabited the stunningly beautiful regions of northeast Spain and southern France for centuries. They speak their own language, celebrate their own holidays and have their own traditions, which included fierce resistance to the autocratic regime of dictator Francisco Franco.
Yes, to all that except for the "believed by sholars to be of Celtic origin", which is completely wrong. Yes, there were Celts in Spain before the Romans got there and the Celts are all gone. However, the Basques were there well before that, and all the evidence suggests that they were there a long time before that. (Just how long a time is hard to say). Linguistically (and in some ways genetically, although there has been more mixing in this regard) the Basques are a complete outlier. They are not related to anyone that we can find. Some people have argued that they may havve been related to the Etruscans of Italy, but since the Etruscans have all been dead for thousands of years it is hard to tell.