Applications folders

Hi friends!

I know that there are two Applications folders:

No 1-the Applications folder that is situated at HD level ... that is ... Macintosh HD > Applications (all my applications are in this folder and are available for every user)

No 2-the application folder that is situated in the User folder ... that is ... Macintosh HD>Users>(Myhome)>Applications (If I want that nobody else could use a particular application, I'll put this application in this folder ... well anyway I never used this folder and actually it is empty)

However today I found another Applications folder by chance.

No 3-This folder is situated here ... Macintosh HD>System>Applications. In this folder I can find the applications bundled with MacOs. OK. No problem.

Nevertheless I find the same applications in the Applications folder No 1. If I use the information window for the Preview app (for instance) situated in the folder No 1, the information window says that this app is in the folder No 3 (not in the folder No1) and this happens with all the "Apple" Applications.

I thought that I found a sort of alias files, but these files show a "size", ... the same size in folder No 1 and in Folder No 3, and I must presume that they are occupying space on my HDD. On the other hand may these apps be a sort of an hard link which is different from an alias, but can it show a size without actually occupying any space? I don't think so.

All in all two questions:

Can anybody explain what I described?

May I remove the No 3 folder? and -as I believe I can't- why not?

Thanks

Alberto