How do I unsync my email

Sounds like the account has been changed to an IMAP account. Check your settings in Mail's preferences under accounts and check out the server name and ports used.

If you deleted and re added the account, that would probably have been the cause.

Yes, a POP account will do what you want.

You can't change the behavior of an IMAP account. You can change it to POP. You will have to delete or disable the account in Mail. When you add the account back in, choose Other account type and put incorrect information on the user or password field of the first screen before hitting continue. This will bring you to a more advanced setup where you can select a POP account. Put the correct info for a POP account in the fields.

You might also go to the gmail webmail site and check settings there. Make sure you have POP selected as account type.

There are some additional settings in gmail that you need to make. Read the following help from gmail on setting up accounts. You will note on the first link the link for recent mode. You will need to enable that so you can use POP between multiple devices in the manor that you want:

Settings:

https://support.google.com/mail/troubleshooter/6323470#ts=1665162

To enable Recent Mode:

https://support.google.com/mail/answer/47948

There's no way they were both set up as IMAP accounts before because you could not have done the things you said you were doing on different devices. IMAP accounts will sync between devices. Whatever is done on one device will be reflected on the other because every thing is actually being done on the server and is just reflected on the devices. If you delete a message in an IMAP account on a computer, what you are actually doing is sending a command to the server to delete the message. In turn, the server completes the task and sends the result back to the computer and is sent to all other devices enabled with the same IMAP account.

Anyway, hopefully you have it solved.