I need my app to receive data from Firebase, how do I do that?

I already have two apps that send photos/texts to firebase, now I want to go up one more step, how do I get a list of information and display it in my app or save in a variable

If you have a tip or a way to open my eyes, I’d appreciate it.

Perhaps start here?

Or here?

Specific questions are where the group can help out. It is always assumed you have looked in all of the obvious places before asking a question.