There is no "file" like you want to see, unless you want to read kernel code.
This is a little oversimplified. See the chapter in Rago & Stevens on Signals in 'Advanced Programming in the UNIX Environment' for a lot more information.
There are two parts to 'default signal handling'. These are built into the kernel.
When the scheduler notices that there is a pending signal for a process, it gives the process the cpu if necessary (a context switch). Next the signal is delivered to the process. The process has a signal mask which allows it to ignore a lot of signals if the mask is deliberately set, two signals SIGKILL and SIGSTOP cannot be ignored or blocked. So the process now has a signal. If there is no handler set the kernel takes the default action for the signal.
There is a list of default actions for signals. SIGSEGV, for instance, terminates the process after it has dumped core. Michael Kerrisk has an explanation of the default actions. This is what I think you want.
The kernel has code to handle all of these signal's default responses for the process.
If the kernel gets a signal for "itself" (depending on what flavor of UNIX we are talking about) the kernel panics, writes a crash dump, and leaves you with headache. - which is not what you want.