It's complicated, naturally. And this list should be considered current only as of today. But it's the reason I chose to make my transfer in Newark and not Europa ....
https://thepointsguy.com/guide/transiting-european-airports-covid/
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Portland, OR (PDX)
305 posts
(Duplicate of a reply I made to a thread under Italy, but relevant here. Is this duplicate allowed?)
The Points Guy article is out of date as of this post.
The Re-open EU site is "An official website of the European Union" and has a travel planner that allows you to check current regulations, including adding a transit. For example, you can plan "Outside EU" to "Italy" and add "Germany" as a transit:
For the above example, I currently get "Changing flights without leaving the international transit area of an airport does not constitute an entry into Germany for the purposes of the relevant regulation." (referring to entry restrictions detailed prior on the page) under Transit by Air.
The above site claims to be updated frequently, but includes the disclaimer the information is not exhaustive.