While the famous International Brigades were obviously on the side of the Spanish Republicans, the Wikipedia page says they also had an allegiance to Comintern. Were the members, whether they were American, German, or whatever else, all leftists, or was it more of an anti-Fascist sentiment? Or did they just want to fight? At war's end, what happened to them, both in Spain and if/when they returned home? Also, were there similar Brigades of foreigners (excluding the Condor Legion) that fought on the side of the Spanish Nationalists?
So I've addressed several of your questions previously in the SCW AMA we held some time back, although heavily focused on the Abe Lincolns specifically. This answer covers recruitment, and this one the post-war treatment a little, and this one briefly looks at the non-Americans. This covers foreign volunteers with the Nationalists. I also maintain a short bibliography for further reading on the topic.