Levels at different heighets

I am developing a vehicle game and i’m trying to get the different levels to be at the same height. It looks like the different levels might have their own environment possibly? I am trying to merge the different levels in my game and get the different levels at the same height without adjusting the XYZ location coordinates. Would you mind providing me step by step instructions to get the different levels at the same height and merge the different levels. Thank you.

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