Using this answer as a base you can add views_field_view to include a view of content filtered by term.
In more details
1. Create a view of terms that includes count of content using each term
- Create a view that displays terms
- enable aggregation
- add relationship Content with term
- add sort content uuid with aggregation type: count (optional)
- add field content uuid with aggregation type: count
- add field taxonomy term
Your view should look similar to the following
2. Create a view that will display the content
- Include contextual filter on the vocabulary of your interest (tags in this case)
- Add any filters or fields that you need
This view will look similar to

3. Add last view as a field to first view (step 1.)
- Download and enable views_field_view module
- Go to view from step 1. and
- add field Taxonomy term: Term ID
- add field Global view ) (made available from views_field_view)
- on the settings find the view from step 2.
- use
{{ raw_fields.tid }}
as contextual filter
Next screenshot shows the changes from original view and settings for view field

You are done
Your view now should display the terms with count and all the content that have used it.