how to get 3d space coordinates from 2d coordinates using python?

I'm having a 2d orthomosaic image and also its point cloud. If I draw a polygon around an object on an image then in the output it should return me the coordinates of that object from 3d point cloud. As soon as I get the coordinates of the object from 3d cloud I want to calculate the volume of that object.Can anyone help me with the same?