How to enable iCloud Shared Photo Library on my iPhone permanently?

I don't know is this has already been asked or not, if so, please redirect me to that post.

Thanks

Recently I bought an Iphone to my GF (16) and I have a 15 Pro Max.

I've set family sharing and also enabled photos between the 2 accounts. on the camera App I've also enabled shared library on both devices but, don't really know why, it reset to the personal library an not the shared one. How do I make it permanent?

On the preserve settings I have Controls Menu, Night Mode, Portrait Zoom, and Live Photo options enabled

Thanks

[Re-Titled by Moderator]

Original Title: iCloud photos Sharing with family