I assume we all realize the rules for traveling to and within Europe are in flux. No doubt the same is true with respect to changing planes at European airports. Still, I think this article is worthwhile in that reading through the various (current) rules highlights issues one should investigate for trips later this year.
The key thing I learned (a point of caution for those ticket-shopping now) is that Amsterdam-Schipol and Paris-CDG are both prohibiting transit passengers from the US whose ultimate destinations are in other Schengen countries. Schipol also prohibits US passengers wishing to transit to non-Schengen EU countries.
I see that there are differences in the format of PCR [<< corrected] tests acceptable at the various airports as well.