How do I tell my iPhone 12 that I have a MacBook?

You will need to add the email account to the Mac. Then take a look at this as far as Continuity is concerned. Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

Geo30 Said:

"How do I tell my iPhone 12 that I have a MacBook?: I want to read and respond to my email on either my iphone12 or my MacBook and both know what was sent and received by each device. This seems very basic idea. What is wrong with this idea?"

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Trying to Get this to Sync?

Seems to me you are trying to get this all to sync. If that is the case, try a new App Password. Your App Password may need to resync - something typical after update installations. You would log in using that password, rather than your official eMail.

As an example, see how Google performs this:

Go Here: Sign in with App Passwords - Google Account Help - Google

This seems very basic idea. What is wrong with this idea?

The idea is sound. Your devices do not "talk" to each other however.

They each independently talk to a server and your content is synced that way.

That's how they work together.

You can't pair an iphone to a Macbook.

What exactly are you trying to do?