Open Google Photos on your phone or in a web browser
Go to the Photos tab or the album you want to select from
Tap and hold one photo to start selection mode
Tap additional photos to select them one by one
On the web, click the first photo, then hold Shift and click another photo to select a range
Use the Select all option if it appears in the menu
If no Select all option is available, select photos in batches
To select all photos in an album, open the album first and use the selection tools there