@Kyrillos Hi,

The easiest way to capture all your active subscribers in one place is to create a Segment of all subscribers. This is how the segment should look:
 

This Segment will capture all of your active subscribers. Because the Segments are dynamic, each new active profile in your account, no matter in which List he’s added, will be added into this Segment also.

Note that if you want to send Campaigns to this Segment, you might want to exclude people who don’t accept marketing. This is how you create that Segment:
 

If for some reason, you want all of these people to be placed in a List, and not in a Segment, you can export the Segment into a CSV document and then import it into the specific List.

Here’s the documentation on how to export a List or Segment:
https://help.klaviyo.com/hc/en-us/articles/115005078687-How-to-Export-a-List-or-Segment

Here’s the documentation on how to import CSV into a List:
https://help.klaviyo.com/hc/en-us/articles/115005078967-Create-and-Add-Contacts-to-a-New-List

Hope this helps!