I am no longer really active on SmartThings, but suggest you learn how the example project works. It was the direction I used for HubiThings Replica and using the ‘glitch’ workspace gives you the required OAuth redirects.
I’ve never actually tried making an OAUTH app, or indeed an API_ONLY app or an API Access app which may or may not be other names for the same thing, I’ve only ever tried simple Webhook SmartApps (using PHP). I can sympathise with your obvious difficulties in getting started. As far as I can see the developer documentation contains absolutely no practical information at all when it comes to the SmartThings side of things. The best you can do is dissect one of the example apps which isn’t necessarily easy if, like me, you find TypeScript/JavaScript code almost completely unreadable and Glitch to be a completely hostile environment.
I’d just be taking wild guesses, but I note that the CLI command smartthings app:create walks you through creating an ‘OAuth-In’ app, which seems to be yet another name for what the API refers to as an API_ONLY app. So it might be that in this case you do need to avoid the Developer Workspace in order to create the correct type of app and use either the CLI or the API directly.