> OpenID Connect (OIDC) is mostly concerned with authentication. On the other hand, OAuth (or, to be more specific, OAuth v2.0) is concerned with authorization.

That's a common refrain and it's quite inaccurate in that it's using a rather unorthodox definition of these terms. It's like the classic "The United States is not a Democracy, it is a Republic", where the speaker reinterprets Democracy as "Direct Democracy" and Republic as "Representative Democracy"[1].

Same goes for "authorization" and "authentication" in OAuth and OIDC. In the normal sense, authentication deals with establishing the user's identity, while authorization determines what resources the user can access.

Open ID Connect does indeed deal with authentication: it's a federated authentication protocol. The old OpenID also tried to introduce the concept of a globally unique identity in addition to authenticating that identity, as the GP mentioned. But OpenID Connect still supports federation: An application (the consumer) can accepts users logging in through a completely unrelated service (the identity provider). I believe this was originally specified mostly with the idea of third-party authentication in mind (using Google or Apple to log in to another company's service or conversely using your corporate SSO to log in to a SaaS web app), but microservices are very popular nowadays, even services that don't support external login often use OIDC as the authentication protocol between their authentication microservice and other services.

OAuth on the hand, started as a method for constrained access delegation. It allowed web services to issue a constrained access token that is authorized for doing a certain set of operations (defined by the "scope" parameter and often explicitly approved by the user). Getting constrained access through OAuth requires performing authentication, so you can say OAuth is also an authentication standard in a sense. But since OAuth was designed for pure deelgation, it does not provide any user identity information along with its access token, and there was no standard way to use it for federated authentication. Open ID Connect essentially takes OAuth and adds identity information on top. To make things more complicated, there have been a lot of OAuth specifications published as RFCs[2] over the last decade, and a lot of them deal with client authentication and explicitly mentioning Open ID Connect (since arguably most OAuth implementations are also OIDC implementations now).

In short, Open ID Connect is quite accurately described as an Authentication standard. But OAuth 2.0 has little to do with Authorization. It allows clients to specify the "scope" parameter, but does not determine how scopes are parsed, when user and client are permitted to request or grant a certain scope and what kind of access control model (RBAC, ABAC, PBAC, etc.) is used. That's ok, since it leaves the implementers with a lot of flexibility, but it clearly means OAuth 2.0 is not an authorization standard. It only concerns itself with requesting authorization in unstructured form[3].

Better examples for proper authorization standards are declarative authorization specification DSLs like XACML, Alfa, Rego (the language using in Open Policy Agent). I guess you could also put Radius in as an example of a protocol that implements both Authentication and Authorization (although the authorization is mostly related to network resources).

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[1] To be fair, the meanings of "Democracy" and "Republic" have changed over time, and back in the 18th century, when the US was founded, it was popular to view the Athenian Democracy as the model case of a pure direct democracy and to use the term "Democracy" in this sense. Over time, the meanings changed and we got a weird aphorism that remains quite baffling to us non-Americans.

[2] https://oauth.net/2/

[3] RFC 9396 is a very recently published optional extension to OAuth that does structured authorization requests, and defines a standard method for resource server to query the requested and granted authorization data.