Selecting multiple photos or videos in Google Photos can be tedious. Usually, you would long press on the first image to select it and then press on other images one by one to select them as well. But there is a much easier and faster way to do this.
Long press on the first image until the blue selection appears, and then slide your finger over any additional photos you want to select without lifting the screen. To select more than what is shown on the screen, slide your finger up or down and hold to automatically scroll and select as you go. If you accidentally remove your finger, you can still add new photos, but have to press them individually.
Google Photos was the first app to implement this feature several years ago, but recently Apple Photos has added a similar gesture. Check it out!