Regarding the rectangles. When you complete the first one, select the face and edges and either make them a Group or a Component. Then, when you create the 2nd rectangle, the geometry (lines) won’t interact with the first rectangle.
And welcome fellow Tiny Home enthusiast!
3DW page is your personal page on the 3D Warehouse site. A place to find, download and collect Components of others, and show all the Components you may have chosen to publish. Here’s my 3DW page, as an example…