The youngest supernova remnants still look like soap bubbles and the gas is hot, emitting lots of X-rays. Older supernova remnants are not as bubble-like . As they cool, they won't be bright in X-rays anymore, and we see them mostly at radio wavelengths. The shockwave from the supernova effects the ISM as well. Shockwave sweeps up material, transferring kinetic energy and heating it up.
Remnant continues to expand, slows down, no longer a shockwave.
Eventually the remnant becomes just another part of the ISM . Hot ionized gas from supernova remnants is one part of the ISM, as is the enriched material once it cools down. Takes up a lot of volume, but doesn't have a lot of mass in it. Recall stars always form in clusters. Will get several high mass stars that form at the same time can get supernovae happening in rapid succession .