How to watch PeerTube videos offline:
- Go to the video on the web or the official app
- Click ⋯ below the video
- Select "Download"
Once it's downloaded to your device, you can watch the video even without an internet connection.
NOTE: This option only appears if the video's uploader has allowed downloads. If you're an uploader you can activate or deactivate downloads at Video > Manage > Customisation > Enable Download (tick or untick, then click Save).