package installation directories

When you automatically download and install packages through the apt or dpkg command , it installed in in a very detailed instructions(i mean perfectly in usr/share and usr/lib); i wanted to know : 1 . how the developers choose the directories and not simply put all of them in a folder;i know it's related to FHS but if you look at the picture below you see they have made a lot of symbolic links to another directories like you have symbolic links in usr/share/firefox-esr for usr/lib/firefox-esr and vice versa; you see symbolic link points to somewhere else , or sometimes points to usr/share (though we could just put the file in the usr/lib); i know it's FHS but no where has any instruction to guide me to this level of complexity; if there is any instruction written somewhere or any book , would be happy to know the link.