Who checks that Residential Building Codes are all met?

[Trace P.]

Hi! It's the first time I am going to be buying investment property in the States. I read about Residential Building Codes, which consist of hundred or thousands of pages and is updated almost yearly, differing depending on state. I was wondering, who in the power team ensures that these codes are met? Will some authority check that the codes are met prior to issuing work permit? Or are all contractors supposed to adhere to the codes by law? If there are no contractors involved, then is it the inspector who is supposed to point out any violations? As the Owner buying the property, are there good practices that I need to check related to Residential Building Codes?

Thanks!