How does it technically save the data when you add a mirror VDEV to an existing pool

I'm curious to know how the data is saved when you have an existing pool with a single mirror VDEV of 1TB to which you later add a second VDEV of 4TB. Does the data you add to the pool get split among the 2 underlying VDEVs or the file is structurally sound and saved by the OS either on VDEV 1 or VDEV2 depending on the space left on each VDEV. The reason i am asking is because let's say i later want to remove the 1TB VDEV and replace it with a bigger mirror VDEV of 12TB, will i be stuck or it will be fine.

Thanks.