How Nintex form auto populates values of some fields based on another field

For Classic Forms such as that, I believe the most common way is via Custom JavaScript. 

To assist you in finding *something* that could give you a hint, I suggest going into the Properties / Settings of one of the Controls being auto-populated. When you're in there, find the "Advanced" expansion menu and click on it:

Once you've expanded that section, look for the Store Client ID in JavaScript Variable, option to see if it's set to "yes". If it is, look at which variable name is being used to store that ID value. In the example below the variable name I'm using is called "DescriptionID":

This is now the part where it gets a little more punishing because the code that is being used to control how that variable is being used could be in a few different places. 

First thing first, exit out of the Control Properties and go to the Form Settings:

Once you're in the Settings dialog, expand the Custom Javascript section:

Inside of the expanded section you might see some JavaScript Code:

And the code being used to control your control's value could be in there. Simply hit ctrl+f to open your Browser's Find tool, and search for the variable name you got from the previous step.

If you find it in the body of code then that is likely where it's being changed!

However if you don't find it there, then you can look to see if there are external JavaScript files being used in your form. To do that, expand the "Advanced" section of the Form Settings:

Here you will see a list of external files that are loaded during the load time of your form. Look for anything listed in the Custom JavaScript Includes section:

If you see anything here, then you'll need to find the actual file at the location listed, and search it for the variable name / control references. 

Before going deeper down the rabbit hole, let me know if you have any luck with either one of those methods.