I was given a an iMac but it’s locked with someone’s Apple ID how do return it to factory if I am unable to get that info

Refer to Activation Lock for Mac - Apple Support.

Excerpt:

"If your previously owned Mac shows an Activation Lock window that asks for someone else's Apple ID, the previous owner may need to remove Activation Lock for you."

The Mac is effectively useless until you do that.

You cannot.

The Mac was not prepared for transfer properly by its previous owner.

Please see: What to do before you sell, give away, or trade in your Mac - Apple Support