The process of making an iPhone app is not fully automated.
At the moment there is no way to fully automated the process of uploading and submitting an iPhone app to Apple for your clients.
The communication with your client will start when they send you their project files. Explained here.
You or your clients should have an Apple Developer Account ( 99$/year)
Your job will be to
Build the app
Submit app for review
Your client should send you an email with the app ID.
Next, in terminal run
cd Mobile App
This script will as you question how you want to publish the iPhone app, on what account. Do this in collaboration with your client.
Create a new app in your iTunes connect portal.
To take screenshot, you can use the in browser preview to take screenshots. There we have options to correct the screens to correct dimensions.
Upload the screenshot in iTunes Connect. Check different requirements.
After you have put all the descriptions, and screenshots you will be able to submit the app for review.