Where is the code for added user registration fields? I've added a few extra fields relevant to my interests to the user registration form. The problem being, I'd like to insert a db_insert function to make use of those fields, but I don't know where the code for the extra fields are. That being said, I'm not entirely sure if altering that code is even necessary.

I'm very new to all this. I'd really appreciate some help, have a good day.
In the field settings, tick "display on user registration form". 

All of the database stuff is taken care of automatically in drupal so you'll rarely have to worry about that stuff unless you are adding your own custom modules that require tables.

What are you trying to accomplish with these tables? Views etc. will have access to them.