Generally on the modern furnaces you'll see the big duct that is the return main feed that comes. from the bottom of the furnace where the fan is. Right between the furnace and where the duct joins it there is a slot where the filter slides in.
Hey we gonna need documentation and signed affidavits to acertain the veracity of the fantasticalness of your hoo haa and wazzoo ;+D
CDC headquaters is not far from my house . Great town I love the big A. Trees everywhere and the colors in spring and fall are awesome. You can do the city thing or get in your car and in 1 hr you're in the mountains