Difference Between 2 Ways of Searching in JSONField

[frozen-yogurt]

Hello based on past issues. I found two ways of searching for JSONField(django.contrib.postgres.fields).
The first way is to override FILTER_DEFAULTS. The second way is to simply use filters.CharFilter to do the search work.
If I have a model:

class Event(models.Model):
  id = BigAutoField(primary_key=True)
  event_time = models.DateTimeField(auto_now_add=True)
  detail = JSONField(null=True)
  
# detail has schema {name: string}

Here is an example of overriding FILTER_DEFAULTS:

class EventFilter(FilterSet):
    FILTER_DEFAULTS = deepcopy(filterset.FILTER_FOR_DBFIELD_DEFAULTS)
    FILTER_DEFAULTS.update({
        JSONField: {
            'filter_class': filters.CharFilter,
            'extra': lambda f: {'lookup_expr': ['icontains']},
        },
    })

    event_name = filters.CharFilter(name='detail', method='filter_event_name')

    def filter_event_name(self, qs, name, value):
        return qs.filter(detail__name=value)

    class Meta:
        model = Event
        fields = { 
            'event_time': ('exact', 'gt', 'lt', 'gte', 'lte'),
        }

Here is an example of just using filters.CharFilter:

class EventFilter(FilterSet):
    event_name = filters.CharFilter(name='detail', lookup_expr='icontains')

    class Meta:
        model = Event
        fields = { 
            'event_time': ('exact', 'gt', 'lt', 'gte', 'lte'),
        }

So based on above two methods, I have two questions:

1. What is the difference between two implementations, in terms of performance and other potential functionalities.
2. For the first method, I found that when I override FILTER_DEFAULTS, the DateTimeField filtering behaves different from the second method. When I GET localhost:8000/api/events?event_time__gte=2019-07-11%2000:00:00, for the first method, I found my SQL is searching my local time. However for the second method, my SQL is searching UTC time. I am still confused about what cause this difference.

Thank you