How to uninstall Windows (bootcamp) bootloader after erasing all disks and restore macOS?
You could not have erased all the disks from recovery, otherwise there would be no Window Recovery to boot to. If you used the Disk Utility to erase all disks visible in the Disk Utility, then the result could be the same as the problem you describe in your question.

In other words, you did not know the Disk Utility does not show all the disks.

The EFI partition does not appear in the Disk Utility. This partition contains the Windows boot files. If you were to erase Big Sur, then the Mac would look for bootable files and would choose Windows if no other choice was available. This boot would not succeed, if you erased the Windows partition. Reseting NVRAM would not matter. Upon startup, the firmware is going to look for an OS to boot, which apparently in your case is the Windows boot files in the EFI partition.
You should be able to boot to macOS Recover over the internet. Occasionally, this can fail do to problems at Apple's end. Using another Mac to make a USB bootable installer is another alternative. Regardless, once booted to an installer, you should be able to use the Disk Utility to erase the internal drive. However, the question &quot;How to change USB scheme when making bootable Catalina installer?&quot; demonstrates that the Disk Utility can fail to work properly when Big Sur has been installed. An alternative would be to enter the diskutil command give below in a Terminal window. When this command is executed from macOS Recovery, internal drive of your Mac should be erased.

The command given below assumes the internal drive is disk0.

disktuil erasedisk apfs &quot;Macintosh HD&quot; gpt disk0