Here in the USA the Lincoln Bde. was looking for a retirement check and veteran benefits? With Biden and Harris they may get it. ?? Sure they were patriotic Americans, only with a little leftist lean. They fought for the Republican side. It was a good war for the Germans and Russians, they got to practice tactics and test weapons for WW II.
The Soviets also got to scam their Republican “allies” out of the Spanish gold bullion supply, as well as use the OGPU/NKVD to eliminate Republican factions that were Trotskyite or otherwise unacceptable to Stalin.
The Soviets learned the wrong lessons regarding the use of tanks and, as the '37 Red Army purges were going on, dismantled their mechanized (combined arms). They also arrested most of the Soviet advisors on their return to the CCCP.