I just bought a Xiaomi air 13.3 (2018). It comes with 2 M2 SSD bays, one is occupied by the stock M2 NVMe (that I replaced with a Samsung 970) and the other one is open.
I tried to put the old SSD in the empty slot, but it doesn't fit. I looked into it and I found that the second slot isn't an NVMe slot, but a mSATA or SATA i think. (I couldn't find the specification while googling).
Since I still have an empty SSD slot, I am looking for a drive between 500GB and 1TB to put into the laptop. The cheaper the drive is the better.