By default - when you customize a session or create something from scratch - your Session will be "Not Published". This simply means that your other admins will not be able to find the Session in Search Curriculum. You can edit and continue to make changes to your Session and you will be the only one that can see it.
Once you are ready to make it available in "Search Curriculum" (only for your church) - you can hit the PUBLISH button in the Session Toolbox. You can always toggle this PUBLISH and UNPUBLISH button.
Note: You can choose to keep things "NOT PUBLISHED" and you can still Share via Easy Links or Share via Email. This mechanism only hides it from Search Curriculum.
Note: Sessions you create or copy/customize will remain only in your church/organization. These will not be available to other churches.