Frankfurt to Buenos Aires back to Frankfurt

but won't I also go through Customs upon arriving in Germany from Argentina

Yes. You will go through customs on arrival in both countries.

To clarify: "Customs" means "rolling your bags by an officer, who might want to look at it to see if you have cigarettes, alcohol, agricultural stuff or other items that are regulated or not allowed"

In contrast: "Immigration" (losely) means "Some government official is looking for at your passport and/or visa" . This always happens on arrival but in some countires also on departure. Germany does exit passport control. I don't know about Argentina.

Please note that you ALSO have your passports/visa inspected during check in by the airline. They will not give you a boarding pass, until they are convinced that you meet the entry requirements. This is separate from "immigration".