In principle, a debian installer is nothing but a scaled model of your system, from the perspective of the root directory. It contains folders, exactly as it is on your system and "projects" the contents of a .deb file on to your system.

There are general rules of where to install what, for example: binary files in /usr/bin/ , libraries in /usr/bin or /usr/share, desktop file in /usr/share/applications and so on (see also here). If you are interested, you should look into a .deb installer to see what it installs where.