Confused

As I've understood it, a proof of vaccination will do. However, if I understand the rules correct, a stop at Heathrow means you have to quarantine upon arrival. https://www.auswaertiges-amt.de/en/einreiseundaufenthalt/coronavirus

Thank you Badger, but if we are only transitioning - meaning not leaving the international transit area - then we don't have to quarantine right?

I'm not sure about that, the rules are a bit complicated (and change every now and then).

I used emed prior to returning from US from Iceland.

I purchased the required 6 tests. I carried 4 tests for 2 of us in case there was a problem.
I used my Pixel phone. It tells you that you need 2 devices, but you don't.
I suggest watching a video of how the test is done prior to doing your test. It is important to leave the card flat on the table and not pick it up.
I had to call in twice for my test because the camera wasn't working properly.
My companion had to call in 4-5 times because he had put his shortened first name in his Navica app (have this app installed and ready to go) and it took a long time for it to update when he changed it to match his passport.

Otherwise it was easy. The delta agent looked at our emailed certificates before she would check us in. After that, nobody checked it.
Let me know if you have more specific questions.

Despite what they say, you can use the 6 tests for 6 different people.

Thank you Vandrabrud for your response. I still don't understand how they get your results? Does the test work immediately and you show the test results on camera? Sorry, for being so lame but want to get this correct.
Arlene