how do I change my iPhone from iMap to pop

POP and IMAP are a function of your email account, not of your iPhone per se. If your provider offers POP as an option for your account, delete your current account from Mail on your iPhone and set it up again as POP following your provider's instructions.

Regards.

Yes, with IMAP accounts, all messages are stored on the email provider's servers, so when you delete a message from one device, it will be gone from all devices connected to that email account. POP messages are stored on the device, not a server, so if you delete a message on one device, it won't be deleted from others. However, it also means that you will have duplicate messages, and mail sent from one device will not appear on any of your other devices. You might want to read this before deciding:

https://www.geeksforgeeks.org/differences-between-pop3-and-imap/

When I refer to your email provider, I mean just that, whoever is providing your email account.

Regards.