Lost trying to use the Zoho Creator API v2

Hello,

I'm having trouble getting started with the Zoho Creator API -- not even sure which client type to register.

I have some business customers that log into my Zoho Creator app as Customer Portal users to add sales information, etc. Some of them have been asking for some time if we are able to give them an API so they can integrate to us. Is this possible with the Creator API?

I'm having trouble understanding the concept behind registering a client app, so my first question is whether each of these customers who want API access would need to register their own client app? And can I link the client app to their portal user email so that they have the same access and restrictions as if they were logging into the portal directly on a web browser? (i.e. which fields are visible, fields with pre-defined values, etc.)

If that's how the API is supposed to work then at least I know I'm on the right track. If so, I could use some help, perhaps a quick paid consultation just to help me get started. Please reach out if interested so I can coordinate with my developer (he's not a Zoho specialist he does other non-Zoho development work for me).