what is APPLE SSD SM0128F Media

If your iMac shipped with a Fusion Drive setup, then you have two physical internal drives. One is a small fast SSD and the other is a large spinning hard drive. A Fusion Drive merges the two drives together to provide a large amount of storage provided by the hard drive while trying to make performance faster by using the SSD as much as possible. Here is an Apple article about the Fusion Drive setup and its purpose:

How to fix a split Fusion Drive - Apple Support

How did you add your new SSD? Is it internal or external? I'm a bit confused by the wording of your original post here since you said you changed the Fusion Drive which implies you changed an internal drive. If so, then you should have seen the Apple SSD -- you can remove it if you want. If you are just booting to an external SSD now, then you can just erase the internal Apple SSD to use it as extra storage (or even ignore it after you erase it).

Edit: If you have not already upgraded to macOS 12.x Monterey, then I suggest you do so before removing the Apple SSD since macOS 12.x Monterey requires an original Apple internal drive the first time Monterey is installed on a computer in order to update the system firmware. After the initial install of macOS Monterey, then any subsequent reinstallation does not require an Apple internal drive to be installed.

It is the internal 128 GB SSD device name (quite small disk BTW -- I guess you do not plan to put much user files on it?).