How did you unbind mail from a steam account like in steam-panel?

Question

Steam

Hello!
For the sake of interest, I am creating the same site as the steam-panel by Mr. Alex
And I have already done everything to check the validity of the account and that's it. But I ran into a problem, how Mr. Alex did the unbinding of the phone, email, etc. change the password. Looked all over, but couldn't find it. Tell me at least how he did it, I'm not asking you for a script. Tell me where to dig!