Open the Settings app
Tap Screen Time
Tap Turn On Screen Time
Tap Content & Privacy Restrictions
Turn on Content & Privacy Restrictions
Tap Content Restrictions
Tap Web Content
Select Limit Adult Websites
To block a specific site, go back to Content & Privacy Restrictions
Tap Content Restrictions
Tap Web Content
Choose Limit Adult Websites
Tap Never Allow
Enter the website URL (or domain)
Tap Done
To block another site, repeat the Never Allow steps
To block sites using a specific app, open the website in Safari, then use Screen Time restrictions to manage access to that content as needed