Which file is Plugin called? Plugin Confusion!!

Hi,

I’m a newbie to wordpress coding and web dev in general.
So I’m trying to modify this hover bar function by ShareThis plugin by adding another button to it, however, I’m not sure how to do so since I don’t really understand the relationship between plugin file/directory and the rest of the theme files.

So when I activate plugin in wordpress admin, what exactly happens and what files are generally involved when doing so other than the files in that relevant plugin folder? (e.g. functions.php, etc)

I’m not sure which file I should go and start editing… A high level description can suffice, please help!!