How are letters signed on mailing services?

Question

SPF

A question based on the example of the getrsponse service.
There you enter the mail From which you want to send letters, a letter will be sent to this mail confirming that this is my mail, and then you can send letters from this box.
You don't need to make any cname,mx, etc. entries, but letters are signed with DKIM/spf, etc.
How does it work?

Answer 1

Through dynamic mail server rules.

Answer 2

The return address when mailing is not your domain, but one of the domains of the mailing service. On the screen is just a letter via GetResponse. Accordingly, their domain is configured for their servers.