To select large groups of photos, you have to enter into the Select Mode.
The Select Mode is accessible through 2 ways:
Either by clicking on the Select button on the Header
Or by long-clicking on a photo
Once you are in the Select Mode, you can
Select photos and videos one by one,
Select them all by clicking on the more button (the 3 dots) and "Select All"
Swipe them to select a large number of them quickly