A quick Google of the same question brings up this:
“Transfer via Frankfurt and Munich: Simplified rules for passengers Simplified rules now apply for passengers travelling to Germany on an international flight with a connecting flight to a Schengen country. Provided that the entry requirements of the destination country are met by the guests, a transfer in Germany (including entering into the Schengen area) is now possible. Prerequisites are a confirmed ticket for the onward flight within 24 hours and/or other documentation for the onward journey. The required proof of a negative Covid-19 test, the vaccination against Covid or the immunity is still required (exception non-Schengen to non-Schengen).
This rule does not apply to passengers from virus variant areas”.
Check with Spain regarding quarantine. My understanding is that the US is still banned from entering EU but Spain may have relaxed rules.