Components and groups can be thought of as containers that hold stuff. Typically raw geometry. Tags are given to or put on components and groups and are primarily used to control visibility of those objects. Tags provide no separation between entities. You can make either a component or group of the geometry to create an object. That can then be copied as many times as needed. If you want to make each object different, you can either first make the components unique and then edit them. If you create a group of the first one and then copy it, there’s no need to use Make Unique to make the copies different from the others. Simply open them for editing and make the changes.
Every object in the model should be a group or component. That prevents geometry from sticking together among other things.
You don’t put materials on tags. You apply materials to the faces in the groups or components. If you have multiple instances of a component, editing one will result in all other instances getting the same change. If you open a component for editing and paint the faces red, all other instance will show that as well.
You can check to see if a component or group is considered a solid by selecting it and looking at Entity Info. If you’ve selected a solid component it will say so at the top of the Entity Info window. To be considered a solid every edge within the group or component but be shared by exactly 2 faces. no more, no less. There are extensions such as Solid Inspector 2 that can help identify what might be preventing a group or component from being solid.