They're not the same. A relative minor has the same key signature. They start on different notes. The B Major scale starts on B, G# minor starts on G#. When you compose a song, you usually use some pattern of chords as the basis, that would usually start with the I chord, therefore writing a song in B Major the first would be a B major chord. As a composer you're using the feel of the key you play in, playing B Major starting on the B will have a very different feel/mood than starting on G# and playing through from there. They all relate, modes are the same way when you get there. You're picking the one that has the feel you are looking for and the relatives are useful for switching from major to minor.
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