When you enter hard mode your world gets two streaks of Hallow and Crimson or Corruption, depending on what it had before. These generally stripe down at an angle and meet under center of the map but can be offset, apparently controlled a bit by where the Wall of Flesh is defeated.
All of the information I could find said that which side got which biome stripe was random. This however does not seem to be the case from my findings and I could find no information that wasn't already 2-3 years old.
So, is this still random? I am up to 37 resets and the only thing I can change seems to be where the Crimson starts but it is always to the right leaving me nearly no surface Hallow on the left of my map (which is why I am going back to try and reset it.)
To address comment conversations below. I am now up to 58 resets on the same world attempting to get the Hallow and Crimson spawns to swap places on my world. What I am looking for is someone with Terraria v126.96.36.199 to have the same world with both configurations of the hardmode V spawn, hallow on left and right. For testing my world seed is: 1486978588 and is a large world, a post about the contents of the world may be found here