Open Settings
Tap Screen Time
Tap Content & Privacy Restrictions
Turn on Content & Privacy Restrictions
Tap Content Restrictions
Tap Web Content
Choose Limit Adult Websites
Under Never Allow, add youtube.com
Add m.youtube.com
Add www.youtube.com
Add youtu.be
Go back to Content & Privacy Restrictions
Tap Allowed Apps and disable Safari if needed
Use Screen Time passcode to prevent changes
If YouTube app is installed, press and hold the app
Tap Remove App
Open App Store restrictions in Screen Time
Turn off installing apps if needed
Use a DNS or router-level filter to block YouTube on all browsers
Set up Guided Access for temporary blocking during use