Sharing is actually pretty simple. If an organization member can create content (based on their license), then they can share that content to others. Here's a link to the documentation on sharing.
Share items—ArcGIS Online Help | Documentation
This blog post describes an upcoming change to sharing. https://community.esri.com/community/gis/web-gis/arcgisonline/blog/2019/11/22/whats-coming-in-arcgis... There's a good diagram in here that conceptually shows how sharing works.
Sharing includes who can access an item--you, your organization, or everyone--and what groups it's shared with. "Private" sharing means sharing to specific groups that contain specific members. If you want to be very restrictive and share with only one person, you'd have to create a group where you and that person are the only members, and share your item to the group.
hope this helps,
Mike