This isn't about a user agent. In basic terms there is supposed to be some kind of software that attests that the browser is actually what it claims to be. On the other side, a server can trust this "attester" or not. So even if you wrote software that always attests what the browser claims to be, Netflix for example could say "nah, I don't trust you bro".
On Android this attestation would be done by the Google play services (afaik). On desktop, the OSs would need to implement this attestation.