Create a key to copy examples filled with your team.
Subdomain or root domain?
A sending subdomain keeps root MX free for inbound.
Yes. Verify something like mail.example.com so the root MX stays free for Google or Microsoft. The sending domain name is the host you verify. You can send from any address at that domain.
hello@mail.example.com and receipts@mail.example.com both work once mail.example.com is verified. There is no mailbox to create. Receiving, if you want it, goes on inbound.mail.example.com.
Verifying the root domain still works. It just makes inbound MX harder, because PostShiba must not take the root MX.