How do you sync your yahoo mail between your laptop and iPhone?

They should work together if they are IMAP (as opposed to POP3) accounts. When you set it up on an iPhone it will be IMAP by default. You can check this by logging in to Yahoo Webmail: https://mail.yahoo.com on your computer, and verify that anything that you do on your phone is mirrored in online yahoo webmail. And you can always use webmail on your laptop. On your laptop if you have a Yahoo mail account set up delete it, then add it back and it should automatically default to IMAP.

You don’t sync between devices. You sync both your laptop and your iPhone to your Yahoo mail account. The Yahoo server has the master copy, and it distributes it to any email clients. Any changes made on one platform will appear on the other.