How to add photos to an existing album in the mobile app

Go to Photos in the main menu (three lines in the upper left corner of the screen)

Go to Albums in the bottom menu

Tap the album you want to add photos to, to open it

Tap the three dots in the upper right corner to open the Album Menu

Select "Add photos"

Add photos and videos from the timeline by tapping them

Tap the check mark in the upper right corner