What do I do if I want users to be able to see/download some assets, but not other assets?
You can control which assets a user can see or download by putting them into different access levels, then controlling by permissions that groups have for those access levels in the Admin area.
For example, if you would like a group of users to be able to download ‘General Use‘ assets, but only have the ability to see some restricted assets, then put the restricted assets into a new access level, and in Admin ->Groups - >Access Levels, give a level ‘3‘ permission to the general use (or Universal) access level and level ‘1‘ permission to the ‘Restricted‘ access level.
Note that if a user is a member of another group for which the permission settings allow the asset to be seen/downloaded,
then that takes priority and the user will be able to perform actions according to that higher permission.