Unlike the free-form nature of custom attributes, standard attributes come from a fixed set mapped in the OAP.
Standard attributes can be added via the following approaches:
Using the Connect API - for buildings and equipment
Using the onboarding system to mass upload for equipment
Individually via onPoint property configuration
You can see the attributes associated with equipment by viewing the specific tab in onPoint.
You will find a series of articles discussing various aspects of setting Attributes, both Standard and Custom, here: