Is this right? Out of curiosity, how do these sites flag themselves as content for adults?

Obviously, this is the solution. Sites that do this correctly, and software that detects the metadata. Parents are the ones that need to choose the correct software and enable the correct settings.

Sites that do not set the correct metadata are legible to be banned by authorities.

I think it is clear that it’s just a matter of time that a faster, more secure, free and open source operating system will win over a worse alternative.