There are say three gmail accounts added in Mac system preferences whose mail address are .
( a ) example1@gmail.com
( b ) example2@gmail.com
( c ) example3@gmail.com
Another default iCloud mail account whose mail address is example4@icloud.com that is also added in internet accounts .
From three gmail accounts one of the gmail address is an Apple ID signed in Mac system preferences , so when you receive a mail from example4@icloud.com and you compose a mail when you click on drop down arrows one of the gmail address say example1@gmail.com that is an Apple ID will be visible .
See the article Set up and use Hide My Email in iCloud+ on all your devices - Apple Support
- In Mail (macOS Monterey 12.1 or later): open a new message window. In the From field, choose Hide My Email in the pop-up menu.
Create and edit Hide My Email addresses on iCloud.com - Apple Support
Change the forwarding address
Every unique, random address you create with Hide My Email is forwarded to the same email address. You can change the forwarding address at any time.
- On iCloud.com, go to Account Settings, then click Manage in the Hide My Email section.
- Scroll to the “Forward to” section, then choose a different address.
- The “Forward to” section is below your list of active addresses and above your inactive addresses.
Note: You can forward your unique random addresses to an email address associated with your Apple ID. To add or remove an address, go to the Apple ID account website.
