A3 - Most likely you are using scatter mode (shortcut: 3). If there is no surface the asset won't show up in this mode.

A2 - Yes, you can make assets and modular assets. Here are some differences:

Just assets

You can make assets that are not snap-able—for example, an organic object.

Snap-able assets

Those are assets you can snap on the grid, and the bounding box fits in Blender Units (or a multiplication/division of it in whole numbers)

Merge-able assets

Those are assets that have the exact same vertices on locations where they meet the other asset. If there are more vertices on one side, the asset won't be mergeable. There is a library called merge-able assets included. You can study this to get an idea of how you can create your own merge-able assets. 

Tip: 

Make your scene completely clean: delete the camera, delete empty collections, purge orphans, etc. Then make some collections (categories and place your assets in the collection. The position of the asset doesn't matter. What matters is the place of the origin. 

Once you've set up your scene with assets, go to Generate Preview in the N-Panel > Modulator. You can use an HDRI, and in that case, you switch on: Use Scene World. Click on Generate and save the file. To load the assets, click on the + button in Asset Libraries. Navigate to your file and click selector. 

A1 - Yes, as long as they are provided in obj, fbx or blend files. Here a few examples you can use: 

• Markom3D's Scifi Modular Kit, and other Kitbash set by Mark. Some are free and others low priced. 
• Pipes Kit by Daniel Grove

Basic workaround to prepare an asset library;

• Load an empty scene, remove everything including the camera. Remove also orphans (purge).
• Make one or more collections like; panels, floors, etc
• Import the objects and place them in the collection
• You can load an HDRI and in Modulator, you go to Generate Preview and check Use Scene World and Use Alpha. Switch over to cycles, and click generate.