Open the YouTube app on your Android device
Play the video you want to loop
Tap the video to show the controls
Tap the three-dot menu
Select Loop if the option is available
If Loop is not available, add the video to a playlist
Open the playlist
Tap the repeat icon to loop the playlist
Use a browser on Android and open YouTube in desktop mode
Play the video
Right-click or long-press the video
Select Loop
Use a third-party app only if needed and only from a trusted source