I see lots of questions of this board re: what the current entry requirements are for Europe, whether you can enter one country from another, etc. The EU is maintaining an official site which you can use to check the current requirements:
On the main page, above the map, is a box with blue outline labeled "Travel Plan." Click on the box and you can enter your departure point (use "Rest of World" if from US, Canada, etc.) and end destination. Click the "Go!" button, then you can add intermediate transit countries.
You can see the current detailed entry requirements for each EU country you are travelling through. For example, I am travelling to Italy flying through Frankfurt in September. Using "Rest of World" for start, "Italy" for destination, then adding "Germany" for transit, I see under Germany specific language noting transit through an airport without leaving the airport transit area is not considered entry to Germany.