Well,
None of the answers could set me free from the pain of specify the single tenant in my single account.
Felt like an alien... Why only myself need to go thru this pain to run a simple command like az login.
I got a explanations - which I greatly thanks all for that - but that's all!
I would be soffering the same pain if I did not update my azure CLI from 2.70.0 to 2.71.0
After upgrading, I now only need to choose on subscription among my long list of ... one subscritions ... to finish my login successfully. But I do not need to specify the tenant anymore.