iMac Mail address locating

Are you asking about this:

That's from the Mojave version of Mail but I suspect it hs not changed much.

If so, that is populated and active when you set up new mailboxes in the left sidebar for sorting your mail. This Apple article shows how to create and use new mailboxes.

Create or delete mailboxes in Mail on Mac - Apple Support

Example: I have one that holds only messages I get from my editor. Once you have moved a few, Mail remembers the senders' address and will offer that option first.

You can also set up rules to automatically move messages the new mailboxes you created.

Use rules to manage emails you receive in Mail on Mac - Apple Support

Just so we're all on the same page let me rephrase you question: For people in your Contacts you want just their name to appear in the To box instead of their email address. Is that right?

Are you sending the email to more than one person? If you are then use the BBC field

to add the addresses that will protect each addressee from the others. Each addressee will see only their name and address in the To box.

If you want just the name of the person in the To box check this box:

Warning: this applies to only your email. It does not apply to the recipients. The only way to hide addresses from recipients is to use the BBC field and do not check this box:

I think you're going to have to expand on that.