Show logged in user name, phone number and email

Hello my friends, I would like to show the name of the user logged in to my app. I’ve already tried without success, I thought about adding the displayName and pointing it to the APP VARIABLE variable in the api. But I couldn’t, could any friend help me with this issue?
I’ve been having this problem for a long time, I would be very grateful if a friend could help me.

Save User Specific Data In Your App - Appgyver And Firebase: