How Windows Work With Users Folder

I like my comp to have only one download location, only 1 temp folder, etc.Regarding users folder I don't know how windows works with these folders, because in every Windows OS I can recall, there have been always multiple folders, even if you set your computer to work with only one user that is you, the absolute and total administrator. Then you can see a default user, a "yourusername" folder, a public folder, an all user folder. Also when installing software sometimes they put their shortcuts into default, sometimes into all and sometimes just in yourusername user folder. Is it safe to delete all folders but the "yourusername" folder? if not, which are the folders you have to keep? Is there a way to make these folders to converge into one? A thing i hate is installing software and not knowing where did it put archives into my computer. I usually check the program files folder, but have seen many times left over folders, some empty some not. Then again the users folder is a mystery to me as if it will keep files from a never used again software forever or not.There is also a Program Data hidden folder.