In order to get custom extensions out of the development system into others you need to transport them.
Transportation consists of export from the source system and import to the target system.
Export
1. Open the extensibility application Export Software Collection
2. This Application shows the currently open transport Collection for newly developed extensions and the hotfix for already transported but changed extensions.
As both are not developer specific but exist for all current developments in the system, you might have to align with developers of other objects on the transportation.
3. In the Value Help for adding items you can use the search to reduce the number of offered objects.
It is recommended to search for the developer name, so that also items that you were not aware of can be put to the collection.
4. Start exporting the collection.
5. Confirm the final pop up for export.
Import
1. In the target system open application "Import Software Collection".
2. Select your collection and press "Import" action.
3. Confirm the final pop up for import.
SAP S/4HANA Extensibility Tutorial