Custom lists are private by default. If a user creates a custom list, it will only be visible to another user if it's shared with them.
If you'd like to share a custom list you created, you can select the three dots on the upper right corner. From the drop down select "Share Custom List".
** Note: Only the user who created the list will have the ability to edit the list. Any users the list is shared with will only be able to view.