Using 3d models

How can I take apart pieces of ready made houses in Sketchup version 8 to use them in my own design? When I select, say, a roof, it’s tricky because you have to select every part of it and it comes with lines attached.

I’m also wondering what the best way to design a house for a beginner would be. For example, to create windows could we just borrow ready made windows of a certain style from the 3d warehouse and just stick them on the walls?

For taking apart models that you’ve found online, if you’re lucky, they’re in groups, otherwise it’s as you described, all edges and faces. I don’t know of any easy shortcuts except to select everything you want, copy it (don’t move it as it’ll come with anything attached to it), group it and then you can move it around as you please.
Look up tutorials on youtube, lots of ways to make a house. Commonly, people draw the floor plan and ‘pull’ up the walls. But play around with sketchup and learn its tools first.