Misc Menu - Pop-Up Collection & Modifier & Object Picker
Is there anything different from gumroad?
There is no difference in the files.
Possible to see the structure and relationship between collection?
No, You cannot view the collection structure and relationships.
Similar to the display in the property shelf-> "View" Tab-> "Collections" Panel.
Can you addon manage collections inside collections?
Yes, It is possible to display and edit collections inside collections.