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https://support.apple.com/en-us/HT201304

Here are some features and settings that you can allow changes for:

Passcode Changes: Prevent changes to your passcode

Account Changes: Prevent account changes in Accounts & Passwords

You can enable restrictions in Screen Time, and not just restricting screentime. You can completely block Siri and dictation for example, or block account changes like the parent comment said.

With Screen Time you can enable Content and Privacy Restriction which allow you to disable a lot of things, like Account Changes or Passcode Changes.

Wow! Used it only for kids so far, didn't figure out that it is actually a hidden security jewel :) Thank you!