I do retail interior designs in detail. For me, I find it works best to tag objects as individual tags and families of objects as tag folders.
For instance cabinetry will be used throughout the store in various areas. So I will have a TAG folder named cabinets. Tags within that folder will consist of Cabinet bodies, Cabinet Faces, Cabinet Panels, Cabinet Faces (includes door and drawer faces as well as fixed faces as a sink front panel.), Cabinet Kick space, Cabinet shelves. (a bookcase is a cabinet body with shelves but without a face).
Countertops have a separate Tag folder. The reason being is that countertops often span several cabinet assemblies. Tags in the countertop folder include Countertops, Back or side splashes (end and side rail), Front drops and counter supports if the counter has no cabinet beneath.
Another Tag folder is Walls. Tags within walls are interior walls, exterior walls, walk-in cooler walls. As Doors and windows almost always are built into a wall, I include those as tags in the wall folder. If you work includes exterior siding or bricks on the wall. I would include those as Tags (objects) in the Walls Tag folder.
I also have a Utilities Tag folder. Within Utilities are Water supply, Drains, Electrical and etc.
The idea is to Tag objects or categories of “stuff” that will appear in many places. These objects or categories will appear again and again in your many projects over time.
Use as many Tag Folders as you need to identify objects that may appear throughout you project.
Now on to Outliner. I use outliner as a map of my current project.
Map the project into areas. Areas are Sketchup Groups of Tagged objects. I may have a Cashier Area. a Refreshment area, a Coffee vending area, A Beer cooler Area, a Pizza Vendor (area), a sandwich cooler, and maybe and auto parts section. Within each of these areas are Cabinets, Countertops, utilities, and other objects tagged in the tags folder.
By having the areas separated in Outliner I can “Turn off or hide” areas I am not working on at the moment.
The Pizza vendor, for instance, won’t care about the Auto Parts section; so I can easily show them just what pertains to their interests.
The owner or general contractor will want a general overview of the entire job. Turn on visibility of the whole works.
The Bricklayer will be interested likely in Exterior walls only.
The countertop supplier will just want counters shown.
In your case show Garage, House 1st Floor, House second floor in outliner as “Sketchup Groups”. Then you can isolate those parts of the building as needed.
In the recent Sketchup releases Visibility can be controlled in both Tags and Outliner. That opened many possibilities for organizing our drawings. As you move on to using Layout you will need to create “Scenes” for presentation of your work. Efficient drawing organization in Sketchup can make Scene preparation a pleasure of a nightmare.