How do plugins store their configuration/field data? I'm creating a custom Commerce License and I notice that I'm handling defaultConfiguration(), buildConfigurationForm(), submitConfigurationForm(), and buildFieldDefinitions(), but I cannot figure out where in the database this information is stored. I know that it must exist somewhere as the data loads up when I view the entity, but I cannot seem to find it. It doesn't seem to have anything to do with Configuration that would exist in a sync folder despite the similar names.

I'd assume buildFieldDefinitions() has something to do with it, but I cannot seem to figure out exactly what that method is doing at all.
Sounds like the answer is "the data is probably the config table... but it could also be anywhere else depending on the implementation." In the commerce_license example I noted, instances of LicenseTypes on Product variants get their own entry in commerce_product_variation__license_type, where the data for that instance is stored in the license_type_target_plugin_configration column. Just like data stored in the config table, this is a hex blob which, when decoded, is the config field data as a serialized array.