I need help with a project

Hi everyone, nice to meet you in this forum. I'm new beginner. I need help with a project. The app is based on a session record where a username and password are requested and a login to verify the user. After this login, the pet's data is requested. The problem is that the pet's data is not linked to the username and password on the firebase. If anyone knows how to do it I would appreciate it very much.

Why not ?