What exactly is a bootleg version?
I’ve read all around that djs like hardwell exploded with their single of a bootleg version. I listened to the original and the bootleg version and they sound extremely similar. Is there a straight definition for this?
A bootleg is the same as a remix, just not officially authorized by the original artist. It was much harder to do before the proliferation of torrents and digital DAW’s.
unauthorized remix
A remix that has not been approved by the author, also used as a term to define a “recreation” of a song, that might be the reason why the one you spoke about sounded so similar.
a little explanation of the ethymology of the word: In the ol’ times it related to people sneaking voice recorders into concert by hiding them in their boots; in the even older times (prohibition in the USA) it was the same, just across borders and instead of sound recorders they bootlegged homemade alcohol 
Also applies to unofficial represses and releases of tunes.
A bootleg doesn’t use stems, I believe, also yea not authorized