Are you using IMAP and deleting the messages at the server or on your phone before opening Thunderbird?

I am using IMAP but I do not know how to stop the emails being deleted at the server (if they are)

IMAP is server based. If you delete a message at the server or from any remote device it is gone everywhere. Are you deleting them from a phone or tablet?

I do not know how to stop the emails being deleted at the server (I do not delete them as far as I know?) I want to stop the emails from being deleted