What watchpoints observe depends on the device & environment. E.g. for ARM Cortex-M MCUs, the (optional but very common) Data Watchpoint and Trace Unit (DWT) provides 1 or 4 watchpoints by using hardware comparators. They operate on raw addresses, since they work at the machine code level. They issue a debug interrupt when the comparator matches the address & operation (read/write), which the debug probe (J-Link, ST-Link, or other such probe) detects & forwards to the debugger (GDB, LLDB, etc.). Hardware watchpoints are extremely useful for debugging memory corruption in embedded systems, since they bypass the code running on the device under test entirely.