In activities, you will see all default and customized activities of all the six components from the leads to tasks according to your filters.
The below screenshot is showing the default filter, users, date range, activities, and the components when the first time you come on the activities page.
Further, you can update filter, users, date range, components and their activities too. In the below screenshot you can see, have updated users, date range, components, and filters too.
Now, in the below screenshot you can see all the changes are been saved as a new filter named test as a public filter.
Updating changes and creating a new filter for them will help in the future to see activities of the particularly created filter when you have already created too many filters.
In the below screenshot you can see, again updating the range in the same filter test, so it would give you an option to update the same filter or else you can save this as a new filter.