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I replaced my fusion drive with a 1TB SSD but didn't break the fusion first. I'm running an iMac (27-inch, Late 2013) 3.2ghz i5 w/ 8GB RAM with Catalina 10.15.3
I'm attaching a photo of my device tree in diskutil.
Is there a way to make the PCI 128GB flash house the OS and be what is usually called "Macintosh HD" and the entire 1TB SSD to be the "Macintosh HDD - Data" Would I be manually creating a fusion drive via terminal of the two SSDs?
In your Container disk3 you have two volumes Macintosh SSD which is a read only volume of about 10GB and Macintosh SSD - Data containing your user files and other writable parts of the filesystem (logs, your installed applications etc). There are also some other small volumes not shown for recovery, booting etc that you can see with diskutil list
These volumes are all part of the OS install in Catalina - see this Apple document About the read-only system volume in macOS Catalina.
While you could try to roll your own fusion drive you'd be adding complexity for no benefit and if either drive failed you'd lose both - see how to prepare two SSDs after upgrade (merge to fusion drive or not, terminal commands)
What you could do is install macOS to the SD0128F disk and use the new SSD as data storage either by manually saving your data there or by moving your whole user folder. The volumes called Macintosh HD and Macintosh HD - Data would remain on the SD0128F physical disk.
