How do i correctly merge 2 edges that are have been mirrored? It's my first time trying to make a car on blender. I have created half of the object and then I have duplicated it. I used ctrl m to mirror it in the y axis. What would the best way to merge the two halves? I have tried to merge the vertices and just adding faces to the gaps but it becomes deformed because of the subsurface division modifier.

Sorry I'm not very good at explaining things.
What you can try is take an empty axis in the middle of your end of the 1st half of the car and mirror it accross that axis using a mirror modifier. (Axis not to be in middle of whole body but at the middle of edge where your car is cut half). don't forget to select the 2 axis across which the car would make sense.