Nether portals teleport me at the same coordinates in the nether as I was in the overworld. When travelling back through a new portal, the game teleports me at the equivalent coordinates in the overworld.

For example:

Location in overworld x: -3000, y: 70, z: 100

Location in nether I get teleported to: x: -3000, y: 70, z: 100

Location in overworld when travelling back: x: -24000, y: 70, z: 800