That lets developers maintain key confidentiality (devs keep their CA private keys) and maintain control over the app installations' access to signed certs (as well as cert lifetime).
Even if it's not a CA, OIDC has some brief words on signing JWT with a registered keypair, which gives a similar, though less robust, ability to keep the private key secret.
No matter what, any of these scenarios still involves figuring out a way to trust the installed app is authorized by the resource owner and the client developer to obtain a signed cert/token (thus shifting real financial liability onto them in OP's scenario). Which probably means requiring the end user to register for your service also, validating the user again rather than the app.
The fundamental fact remains that the human mind remains the only truly secret place, which is why passwords aren't going anywhere, and why DRM solutions have to rely on making it illegal to attempt to obtain the decryption key embedded in device, or making attempted recovery involve physical destruction of the key.
1) You create a "normal" client in Google Developer console (i.e. a web client) 2) You create a native/Android client in the same project. This client is shared across all phones. 3) You add a scope of audience:server:client_id:$NORMAL_CLIENT_ID to auth requests from the mobile. 4) You get back token minted for the web client, from the native client!
This is how it works:
https://developers.google.com/identity/protocols/CrossClient...
The reason it is safe is because you can only do the cross client stuff from a mobile client, which disallows any redirect urls except for localhost and a couple of other special URIS (see https://developers.google.com/identity/protocols/OAuth2Insta...)
It's ok that the secret is not really secret because it's not possible to use it to making a Phishing site since the redirect URL is localhost.
I guess that doesn't answer your "how does it identity the app developer" but it does tell you how these things are deployed at least, and the important fact that there's just one client (not one for every device)
At the end of the day though, everyone has to sign their apps with certs that are pretty well validated. So, it really cuts down on funny business like you mention.
They suggest using PKCE (challenge-repsonse) https://tools.ietf.org/html/rfc7636 to authenticate clients that can't be trusted with a client secret.