What does not need a key
A handful of endpoints are public because they are how an account comes into existence in the first place: registration, e-mail confirmation, sign-in, token refresh, the password-reset pair, and the OAuth entry points. Everything else requires a key.What a missing or bad key gets you
A request with no credentials, an unknown key, a disabled key or an expired one is refused with401. There is no partial access and no anonymous read: the tool catalogue, the endpoint list and every object behind them all require a valid key.
Sessions
The console signs people in with sessions rather than API keys, and those tokens travel in the sameAuthorization header. See Sign-in and account security if you are building against that flow instead.