installing applications

[mith]

Hello all,
being a windows and linux user until a month ago, I still have some problems with the organization of the applications and their files... It's not totally clear how they are installed on the system.

Here are my questions:
When I download an application, such as Safari, it simply creates an icon... Where is the software installed? Where are its files?
I had installed Safari v73, recently I manually downloaded V74... I removed the icon in the Applications folder and moved the newly installed in its place (and had to recreate the link on the dashboard). Now I checked for updates with the automatic software and it says that there's Safari V74 available!! How do I make it understand that I already installed v74? How to clean up the mess I'm generating? =)
And finally, how do I add an application to the StartupItems? Specifically I would like to have Stickies start automatically.

thanks,
mith