Open Google Photos on your computer or in the mobile app
Go to the Photos tab or the album you want to select from
On computer, click the first photo
Hold Shift and click the last photo to select a range
On mobile, press and hold a photo to start selection mode
Tap additional photos to select them one by one
Use the Select all option if it appears in the menu for the current view
For large libraries, use the search or album filters first, then select all within that filtered view
After selecting, choose the action you want from the top menu