An automatic update is triggered in Infuse when the app is opened, as well as periodically (IIRC every 15 minutes)
Automatic updates take place while on Wi-Fi, and if your device is not in low power mode
Manual updates can be triggered at any time using the refresh icon on the home screen or the ‘Scan for Changes’ button in Settings > Library.
If new videos aren’t appearing in your library even after a manual update, it’s possible they may be in folders that have not be included in Infuse’s library index. You can check which favorites are included in these scans in the Settings > Library menu. New favorites can be added via Settings > Add Files > Share Name, then tap the star icon next to the folder you wish to add as a favorite. Adding a folder as a favorite will also cause Infuse to include all files/folders included inside it.