How do I disable this prompt? Every time I boot my phone, I get a message about not having a cellular carrier.
I only use my phone for gaming, so I select "No". Selecting "yes" leads me into something where I might select a carrier, but I cannot fill it out, because I do not have a carrier. The app launches automatically but is not obvious what it is called. The phone's OS is Android version 7.1.1. The phone is a ZTE N9560.

How can I permanently disable this message?
Where are applications that start on boot configured / stored?
The files that gets executed during booting process are definitely located in the root folder and also some of the files under system/xbin folder....

You can't access both root and xbin folder unless you have administrative permission (clearly speaking unless you have rooted your phone) 

If your phone us rooted you can access the folders using root file explorer or android terminal and delete the files which are creating problems.