Ive, noticed that in some evolves maps, or maps in general that involves massing, the units spawn under another unit such as a science vessel. How is this done?

You can't be serious. You do move location to move a location on the Science Vessel and then create unit.

If the unit is a flying type(e.g. Terran Wraith) , just use a Move Location trigger onto the flying unit and create a unit under it..

If it's a ground unit (e.g. Terran Marine) , use a Move Location trigger onto the ground unit, if the unit you are trying to create cannot be put on top of the unit, it'll probably misplace, you can use burrowed units and powerups to put under ground units but powerups will misplace your unit. ;D.