Big Sur Boot Volume Disappeared After Boot Camp Resize

If you used a Windows app to resize it, you destroyed it. You should only use Bootcamp Assistant to modify a Bootcamp partition.

It probably split the Fusion drive. There is no way to nondestructively recover.

Will it boot into Internet Recovery?

What is listed below will Destroy ALL DATA on the drive and there are No Do Overs - period.

To perform this action will require booting from a Bootable Installer. This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Once that is done read on for preparing the Destination computer  >> Special Notation - works on Intel Based Apple Computers & on Apple Silicon M1 CPU

Extra Special Notation regarding the Touch ID equipped Apple Computer. About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

>>>>>>> How to fix a split Fusion Drive <<<<<<<

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives.

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

It isn't recognizing your SSD. Perhaps Windows killed it somehow.

In Terminal, while booted into Recovery or the bootable installer, run this command and post the output:

diskutil list

I don't know what you can do to get it to recognize the SSD. Maybe try SMC reset.

To Quote @Barney-15E " You should only use Bootcamp Assistant to modify a Bootcamp "

How to fix a split Fusion Drive and read all the details completely

It appears whatever tool you used completely destroyed the directories of the SSD to the point where the Mac cannot even recognize it. I don’t know how to fix that.

It looks to me that might be your SSD.

You could try creating a GPT on it and format it ExFAT. You might not even need to format it. Just put a GUID Partition table onto the drive.