Some plugins may extent the Jenkins API with additional functionality. To [blindly] determine if this is the case for a particular plugin of interest, first enumerate the list of installed plugins:
Then, find the plugin in the list and get the value from its “shortName” field:
That value is the name used in the API URL. In this case, that URL would be
https://<jenkins server>/gerrit-trigger. This might be enough to get you what you need, though, depending on the plugin, you might have to reverse-engineer the plugins’ sourcecode in order to find which further nouns/entity subpaths are available from here. You might do a Google search and then grep for the paths that you find from that in order to discover siblings.
This is not meant to be a complicated post, though the information is made complicated by the effort required to find the information.