Every major client has a right there in the message way to do this, so rather than retyping, the shortcut per client:

Outlook: right click the sender's name in an open message or the name in the From line and choose Add to Outlook Contacts, which opens a contact card prefilled with the name and address for you to save. In newer Outlook and OWA, clicking the sender's name shows their card with an Add to contacts option. The address and name come across automatically, you add anything extra and save.

Gmail: hover or click the sender's name and the contact card appears with an Add to contacts option, saving them to Google Contacts and Gmail also auto saves people you email to Other contacts, so a recipient you have written to is often already findable there, promotable to a full contact.

Apple Mail: click the sender's name or the arrow beside it and choose Add to Contacts, which saves them to the Mac's Contacts, and on iPhone tapping the sender offers Create New Contact or Add to Existing Contact. The shared address book means they are then available everywhere the account syncs.

The auto collection worth knowing across clients: most mail systems quietly remember addresses you correspond with, Gmail's Other contacts, Outlook's suggested or auto complete list, so a recipient you have emailed is frequently already stored for autocomplete even without deliberately adding them and the deliberate add just promotes them to a proper contact with a real card. For someone you will write to repeatedly, the deliberate add from the message is the two click habit worth forming, capturing them the moment they first appear rather than retyping later.