Integration with mobile app

Content

Hi experts,

I really need your help.

I have an already existing mobile app to approve quotations.

My client doesn't want to change this app.

I know how to queue approving in the app.

My client also wants the approval to be done either by the app or by the process form.

My problem is how to send back the approval to OIC.

How can I identify which process instance I am approving?

How can I identify the user that is approving once the app cant access the login password?

I know I could implement an OAuth flow to authenticate the mobile client on the OIC, but how to identify the user that is approving?