Email not syncing between IPhone 14P and new Mac Book Air, why?

Emails do not sync between devices, rather both devices should be syncing with your email provider's server. If that's not happening, first step is to delete the accounts from the devices and re-add them.

Just to confirm though, by 'not syncing' do you mean showing different emails, or that there is a time lag between them, e.g. a new email shows up on one device then some time later on the other. If the latter, check the account settings for the refresh interval, it may be set to push on one device and a 15 minute pull on the other, for example.