OAuth2.0 Flow

sequenceDiagram
    participant User as User
    participant App as Client
    participant Auth as Authorization Server
    participant Resource as Resource Server

    User->>App: Click "Login with OAuth"
    App->>Auth: Redirect to /authorize
    Auth->>User: Present login screen
    User->>Auth: Login & approve consent
    Auth->>App: Redirect with auth_code
    App->>Auth: POST /token (auth_code)
    Auth->>App: Return access_token
    App->>Resource: GET /resource (access_token)
    Resource->>App: Return resource data
    App->>User: Display resource data

sequenceDiagram
    participant User as User
    participant App as Client
    participant Auth as Authorization Server
    participant Resource as Resource Server

    User->>App: Click "Login with OAuth"
    App->>Auth: Redirect to /authorize
    Auth->>User: Present login screen
    User->>Auth: Login & approve consent
    Auth->>App: Redirect with auth_code
    App->>Auth: POST /token (auth_code)
    Auth->>App: Return access_token
    App->>Resource: GET /resource (access_token)
    Resource->>App: Return resource data
    App->>User: Display resource data

Google の場合

ref. Using OAuth 2.0 to Access Google APIs  |  Authorization  |  Google for Developers

OAuth 2.0 vs OpenID Connect

下記の動画がわかりやすかった。