How to remove an iPhone from Apple ID without data loss

You should remove the iPhone from your Apple Account, as explained in this related article, then give it to your partner and let him set it up with his Apple Account, using the latest backup he made with his old device.

This is the only way to keep your accounts separate.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you want to share apps and other purchases, set up Family Sharing:

How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support

You can delete the phone from your account but what you don't say is anything about having erased the 12 before giving it to your partner. If you did not then what has probably also happened is all your data on the 12 merged into his account when he signed in. He will have a big task going through all his files and settings and deleting things from your account. This won't affect your account, nor can you do it remotely even if you remotely erased the phone. The data could have already have made their way into his iCloud to which you have no access..

What should have been done was as instructed in -> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you did not delete your data from the phone then you can do so now but it may already have merged into his account. At the very least change your passwords on everything.