Transiting through DOH
 

I can't find a definitive answer on this. Can I transit through DOH and stay airside without COVID tests? I'm vaccinated.

Unless you have are flying to Doha from Bangladesh, India, Nepal, Pakistan, Philippines, Sri Lanka, Vietnam; you wouldn't need a COVID test to transfer Doha regardless of vaccination status.
Although, the COVID requirements of your final destination would still apply.

Your final destination country may still consider that you have visited Doha, even if you only stayed airside.

I'm entering Greece, and Qatar is on their green list same as US, so I guess it will be ok. Thanks.

Yes, if you are flying USA -> Doha -> Greece your vaccination certificate is enough.
No need for a COVID test.

IST Flyer,
I had posted a thread in mid-June asking if a US resident and citizen can transit to Greece through Istanbul (staying in international side) and not be subject to restrictions and quarantines in Greece so long as otherwise they met Greek entry requirements.
I know the Istanbul airport has no COVID requirements for transit passengers.
The conclusion seemed to be that transit through the Istanbul airport to Greece was fine so long as you stayed airside.

Can you confirm?
Thanks

You would be able to fly to Greece when transiting Istanbul as you're a US citizen & resident but I'm not sure 100% if quarantine/self-isolation requirements would apply in Greece; however, most likely not.