Integrating API with my own system

Hi, I have an issue that i don’t know how to start with:

I want to create a system whereby this system will be centralized; users can login to this system and control their hubs from there (adjust temperature, on/off light, etc, etc). We are still in researching on the rest of the brands of the devices

Smartthings happens to be one of the devices that we want to integrate with.

Problem is, I do not know how to go about it so that users can control smartthings from our own centralized system.

Here’s what I’ve done so far, I manage to create a smartapp under my developer account and able to control the devices via REST API. I also read up the documentation on the capabilities of each of the sensors and the relevant API too.

However, I do not know how to let the users control their own smart things since these end users might not have a developer account at smart things, and to be able to control the devices, you need the smartapp to do it (correct me if I’m wrong on that issue).

I have thought of using oauth for this purpose, but that requires the user to have an account with smart things. Also, I do not know the url to login and get the access token with oauth client/secret key.

Please advice me on how to go about doing these

thanks in advance