Where does Linux set the default values for SHMMAX?

The __init function ipc_ns_init sets the initial value of shmmax by calling shm_init_ns, which sets it to the value of the SHMMAX macro.

The definition of SHMMAX is in <uapi/linux/shm.h>:

#define SHMMAX (ULONG_MAX - (1UL << 24)) /* max shared seg size (bytes) */

On 64-bit machines, that definition equals the value you found, 18446744073692774399.