How do I resolve 'This device is already associated with different account' issue after logging into my husband's Apple TV+ on my iPhone?

My husband got a free AppleTV+ account from TMobile, so after ours, in my name, expired, I logged into his on my iPhone. That was find, until I went to download a new app, and it tried to get me to login under his email and my face ID didn't even populate.

Now I woke up to a message on my phone, saying, "This device is already associated with different account" and asking me to "cancel" or "transfer".

I still show as logged into Apple in the Settings on my phone.

1- If I transfer, is that transferring everything over to his account, all my apps, etc?

2- How can I be logged into his Apple TV+ but still be the "owner" of my phone and my apps?

3- Will adding to my "family sharing" resolve this issue?

[Re-Titled by Moderator]