I found a backtrack and it says it's in Em, but isn't it up to me to decide what key it's in? Like couldn't i play Emaj, E Dorian, E Phrygian, E Lydian, E Mixolydian, E Aeolian, or E Locrian over it if i wanted?
Modes aren't keys, keys are either minor or major.

Yes, if it's in the key of E minor then the logical choice is to use the E minor scale. If the chords suggest it then you can look at playing around with any of the parallel modes that may suit it but E minor is always going to fit.
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I hate it when someone on youtube asks what key a song is in and someone replies back with E Harmonic Minor. That just irks me.
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