if I replace my imac 2012 hard drive, will a non apple one work as well? Also, will I be able to download the newest iOS?l

A 2012 iMac will not be able to install the newest macOS or any macOS beyond Catalina, with or without a new hard drive.

Also see the following from Identify your iMac model - Apple Support

iMac (27-inch, Late 2012)

Model Identifier: iMac13,2

Part Number: MD095xx/A, MD096xx/A

Newest compatible operating system: macOS Catalina

Tech Specs: iMac (27-inch, Late 2012)

User Guide: iMac (27-inch, Late 2012)

iMac (21.5-inch, Late 2012)

Model Identifier: iMac13,1

Part Number: MD093xx/A, MD094xx/A

Newest compatible operating system: macOS Catalina

Tech Specs: iMac (21.5-inch, Late 2012)

User Guide: iMac (21.5-inch, Late 2012)