It would be helpful to have diagrams for the OAuth flows. Here is an attempt (websequencediagrams.com code)
title OAuth User Agent Flow

participant "End User" as eu
participant "User Agent" as ca
participant "Apigee" as ap
participant "Login App" as lap

eu->ca: <>

ca->ap: /oauth/authorize (client_id,redirect_uri,scope)
ap-->ca: http_redirect (login_form,id)

ca-->eu: login_form [using native browser]
eu->lap: user login

lap-->eu: permission form [OAuth scopes]
eu->lap: user permission
lap->ap: GetAuthCode /oauth/authorizationcode\n (client_id,response_type,scope)
note over ap
Apigee will generate an access code\n or can be passed one. Authcodes \ncan have attributes like userid
end note
ap-->ca: http_redirect (auth_code)
ca->ap: GetAccessToken /oauth/accesstoken\n (auth_code,redirect_uri) \n+Header Authorization: Basic [base64 consumer key and secret]
note over ap
Apigee will generate token.\nAttributes set in authcode will \nbe copied over during token generation
end note
ap->ca: AccessToken

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