Solid conductive metals will efficiently reflect microwaves like a mirror. That reflection prevents the vast majority of the energy from entering the body of the metal.
Metal compounds will partially reflect, transmit and absorb microwave energy depending on the conductive, dielectric and magnetic properties of the material.
The shape and size make a difference to the pattern of reflective scattering. Microwave antennas and waveguides are made from metals.
Metal sheet will screen things from microwaves so long as the hole dimensions are small compared with the wavelength.