How much of the actual launch pad survives the heat and pressure of launch and is capable of being reusd?
The people who build modern pads, know the power of the rockets.
The flame trenches are designed to take the engine exhaust, use water to protect it from heat and sound damage.
There are umbilicals that are much softer, and are often easier to replace than try to harden. SpaceX learned this the hard way, through trial and error on their first few launches. They had to toughen up parts of the pad they thought would not need it.
Thus a properly designed, modern launch facility will have most of its fixed structures survive with minimal work required, and possibly need some softer elements simply replaced.