So, when you boot up the iMac, it boots to the partition containing Windows 10. Is that accurate?
In "normal", Mac OS situations, when one boots up their machine, holding the Option key while booting will produce a screen containing OS bootable volumes/devices. I'm unsure what would happen in your situation. It would seem strange that the previous owner would have only Windows on the iMac. Typically, one would partition their internal drive, and get that additional partition "ready" for the Windows OS. There would still be a least one other partition with the Mac OS on it.
Maybe someone else could "chime" in here about how to "determine" if there is a version of the Mac OS on your machine, and how to get to it.