So, I installed Windows 10 on iMac mid 2011. This is unsupported, but I did it. And, what I did was install it over the main partition. Now I do not know how to go back to macOS. Any help? I beg you to help me! I tried booting a USB with High Sierra installation, but it is not showing up!
If you installed over the main partition, then I would assume you will not be able to go back and boot macOS. You will need to reinstall macOS. Windows should be installed to BIOS boot. If you did this, then you probably will have to erase the entire drive before reinstalling macOS. If you installed Windows to EFI boot, then you will not have to, but you should since the Window Support Software was designed for BIOS booting Windows.