Can I connect my ipadpro to a very old iMac desktop computer simply to use the larger monitor display? If so how do I display it on my imac

Connect the iPad to a USB port on the iMac. Start the QuickTime Player application (Applications folder) on the iMac.

Click "New Movie Recording" in its File menu. Click the ⌄ to the right of the red record button and select the iPad as Camera.

Click the green button at the top-left of the window to go full-screen.

You cannot. Your iMac is a computer, it is not designed to be a display for an another device. You may want to search third party solutions however please don't expect great performance.