Screen Time actually works well if you configure it properly. You can block the YouTube app and also restrict youtube.com in Safari under Content Restrictions. That combo usually does the trick.
The most effective method is blocking both the app and the website. Kids often bypass app blocks by using browsers, so make sure Safari and Chrome are covered in Screen Time.
For long-term control, combine Screen Time with restricted app installs and a Screen Time passcode. Without the passcode, kids can’t undo anything even after updates.