Repurpose Fusion Drive - Booting from USB 3.2 Gen 2 SSD iMac Retina 4k 21.5&quot; (2019) 3GHz Core i5 with 8 GB RAM and 1TB Fusion drive.
Big Sur 11.2 booting from external 1TB Samsung T7 SSD, backing up to a NAS.
Questions:

Is the internal SSD (28GB?) active in this setup?
Can the Fusion drive be formatted and Windows 10 be installed on it using Bootcamp?
Can the Fusion drive be 'split' and the internal SSD 'fused' to the external SSD?

Thanks for any input.
Is the internal SSD (28GB?) active in this setup?
Your Mac was shipped with an 1 TB SATA HDD and a 32 GB NVMe SSD. So, you have two separate internal drives which were configured as a Fusion drive with macOS installed. You state in your question that you have an 1 TB Fusion drive, so I assume the internal SSD is active as part of the Fusion arrangement. You should be able to verify this by inspecting the output from the command diskutil list.

Can the Fusion drive be formatted and Windows 10 be installed on it using Bootcamp?
Here, I assume you mean using the Boot Camp Assistant to install Windows 10. In this case, the answer is no. You would be trying to install Windows 10 on a USB drive and the Windows 10 installer GUI would not permit this to occur. However, you can use the Boot Camp Assistant to download the Windows Support Software (the drivers), then you could install Windows 10 by using the Windows 10 installer CLI. For example procedures, see this answer.

Can the Fusion drive be 'split' and the internal SSD 'fused' to the external SSD?
You should be able use the internal SSD and external SDD to create a Fusion drive arrangement. You could even install Big Sur into this Fusion drive. I was able to verify this part of my answer by creating this arrangement in a VirtualBox virtual machine. I assume if this is possible in a virtual environment, then the same configuration would work with real hardware.