How do I find or set the apropriate form for base scheme fields?
You can always create a field with the desired settings in the field, widget and formatter settings in the UI. Then export the configuratio and use the settings from there. The field settings are in two files:


field.storage.entity_type.yourname.yml
field.field.entity_type.bundle.yourname.yml


For base fields, you can merge the settings together into a single array.