Mail app: multiple recipients - usage not intuitive

When adding more than one recipient (from known contacts) to the recipients list I’m stuck many times. My usage.

  1. type first recipients name - proposals appear - I set the check mark to the one desired
  2. a) then what? typing comma results in having the typed letters only in the “to” field plus the comma
  3. b) tab brings me out of the context

What is the proper way to add multiple recipients? It is (at least to me) far from intuitive.
I actually start using different programs when the mail should go to more than one recipient.

Yes, this currently sucks:

I remember versions (~2 yeas ago or so) where this was no issue and multiple recipients were selectable easily.