OpenStack Horizon dashboard How choice field or dropdown get populated

How flavor choice field or dropdown of create instance UI get populated

1)
https://github.com/openstack/horizon/blob/master/openstack_dashboard/dashboards/project/instances/workflows/create_instance.py

class SetInstanceDetailsAction(workflows.Action):
    flavor = forms.ChoiceField(label=_("Flavor"), help_text=_("Size of image to launch."))

    def populate_flavor_choices(self, request, context):
        ----
        return flavor_list

2)
https://github.com/openstack/horizon/blob/master/horizon/workflows/base.py

class Action(forms.Form):

    def __init__(self, request, context, *args, **kwargs):
        self._populate_choices(request, context)
       
    def _populate_choices(self, request, context):
        for field_name, bound_field in self.fields.items():
            meth = getattr(self, "populate_%s_choices" % field_name, None)
            if meth is not None and callable(meth):
                bound_field.choices = meth(request, context)