2011 iMac only boots to Windows 7

Good day,

I have purchased a second-hand 2011 iMac 21.5" (A1311 EMC 2428) few days back and it only boots to Windows 7. When I click the Boot Camp icon and choose to restart in macOS it displays "Could not locate the OS X boot volume."

I cannot go into Recovery Mode or Internet Recovery Mode. I tried to reset the PRAM/NVRAM but no result (I also swapped the NVRAM coin battery).

I made a bootable USB with OS X Lion on it (I believe it was the OS X that came with this model of iMac) but the iMac does not seem to detect the flash drive.

When I hold the Option key while booting, it only shows my Windows 7 partition and nothing else (no Recovery partition, no Macintosh HD).

I swapped the Barracuda hard drive with another hard drive and upon booting it brings me to the Windows 7 "No bootable device" black screen. Then I tried to boot into Safe Mode, and it showed a white screen with a grey folder with a question mark on it.

I also swapped the hard drive SATA cable, and put everything back together with the original hard disk but still boots to Windows 7.

I believe that the Mac OS partition was corrupted when the previous owner of this device installed Boot Camp (and the previous owner does not remember which version of Mac OS was is installed by the way). I want to wipe this iMac clean and get rid of Boot Camp and install Mac OS on it.

Could someone help?