It says disk1 is synthesized and it's listed under disk0 as well as the APFS container. You just have a 256GB PCIe SSD, no spinner currently.Not sure why I am seeing a disk0 and a disk1 but this is the info terminal shows
As far as internal upgrades go, take a look at OWC to see what's possible for your particular machine. They're at macsales.com.