... If that is the case, does UL 50, 508, 508A have a standard for designing and building industrial control panels? ...
To address just this point (because augie47 already covered the rest);
UL50 is
JUST for the enclosure itself, it has nothing to do with what goes inside of it.
UL508 is the over-arching standard for Industrial Controls, which covers not only all of the components USED in a control system that are NOT covered in other UL standards (such as UL50 for example) but is also further broken up into sub-sections relevant to what those are and how they are used.
UL508A is the sub-section of UL508 that pertains to customized control panel ASSEMBLIES, meaning what a "panel builder" would make for a specific job or an OEM, using components listed under UL508 and any other relevant standards (again, such as UL50). UL508A can only be applied by a registered entity that has been trained and evaluated by UL to follow and adhere to their standards, and are regularly inspected to maintain compliance. This is why, as augie47 said, MOST of the AHJs out there will look for a UL508A label and accept it.
UL508B is an oddball section rarely used by most of us because it covers specialized relays used in IT equipment
UL508C is what covers Power
Conversion Equipment, such as drives and power supplies.