Plugin handler to register custom field types to be supported

Problem/Motivation

Been looking for way too long at the code already :) How does one define a plugin handler for conditional_fields and actually make it work for a certain type of field. Sar far I've been able to define one in a custom module and observe it is defined, also I see that the plugin for entity reference autocomplete fires on an entity reference field but not my plugin. This makes sence but I simply cannot find how the module actually does this under the hood. So mark this as a support question on coding plugins for this module that hopefluuly is to use to others looking to do the same.