How do you fill in a space between two objects and leave the middle open? I'm not 100% sure how to word this so ill try to simplify it. Lets say you have 2 circles. One circle is inside the other like a doughnut. How would you fill the space between the 2 circles while leaving the middle empty if the two circles have a different number of vertices?
If you have the circles as separate objects, first you need to join them into one mesh by selecting both and using CTRL+J shortcut.

Then switch to edit mode, select both circles, then hit CTRL+E and select Bridge Edge Loops from the menu.