Apple doesn't make any hard drive enclosures, but your iMac uses a standard 3.5" SATA hard drive, just like you'd find in any regular PC.

Here's a link (Amazon US) to a decent drive enclosure that will work with the hard drive in your iMac. It comes with the power supply that's needed since you have a desktop hard drive. If you don't want a full enclosure you could get something like this that just connects to the SATA power and data ports on the drive.

Connecting the drive to a Windows PC: do you know what version of macOS your iMac was running? Unless it was upgraded with an SSD at some point it should still have a mechanical hard drive (SSDs were offered from the factory, but they were quite expensive, so most came with mechanical drives), and if it has the mechanical drive it'll be formatted as Mac OS Extended (Journaled).

Windows cannot natively read this format, but you should be able to use something like HFS Explorer or another similar program to access the drive on Windows.

Take/break apart the Seagate enclosure the HDD is in and then plug the HDD directly into your new enclosure/adapter.

Maybe return the adapter you got and get one of these instead.

ORICO Toolfree USB 3.0 to SATA External 3.5 Hard Drive Enclosure Case for 3.5 SATA HDD and SSD[Support UASP and 8TB Drives] https://smile.amazon.com/dp/B00GAML7OK/ref=cm_sw_r_cp_apa_i_MRyPCbYEVW8SX