I'm advocating for making it obvious when someone is and preventing data processing of individuals without their consent (that includes google photos ML/face recognition).

You can design and manufacture many illegal things and yet that's not common practice because if you get caught you'll get punished. E.g. you can make bombs at home and yet I don't see bombs being common outside.

In the scenario where you want to make your own camera you'd need to e.g. have to have a bright red led when it records. Or the camera modules being sold will already have that. That's it. Hiding one would be get you fined if used. Today you can physically put a recording device in someone's home, that's entirely illegal where I live and yet people just don't do that.

As for knowing you're always being recorded - unless you're a method actor or put in the effort you won't always keep that in mind the entire time you're outside. But I bet if a TV crew points their camera at you, you'll behave differently (or at least your awareness will change)