The control switches supply 12V to both the NE555-based blinker circuit and the external LEDs. A diode on each switch acts as a wired-OR arrangement to supply +12V to a LM7805 (yes, I know they're ancient) which in turn drives the NE555.
The regulator helps stabilise the power rail so that the indicators don't stop blinking or change frequency if I hit the transmit button on my radio transceiver. (In theory… in practice, induced RF does this anyway… I'll have to fix that one day.)
The NE555 drives the gate of an IRF540N MOSFET, which supplies the connection to the 0V rail for the left and right hand indicators, and the buzzer. This is an open-drain connection, and so if you want a flashing tail light or head light, you can connect to the MOSFET drain, the "blinker sync" signal, instead of 0V to achieve this. (This is why the "running lights" switch also powers the NE555.)
The circuit has been in use now for over 5 years, and it has rarely given me trouble. Usually, if there's a fault, it's because something outside the controller itself has developed a short.
Control switches use a simple single-pole single-throw illuminated switch, I have two, one for left and one for right, mounted at opposite ends of the handlebars. The controller itself mounts to the top post using zip ties, and so switching the lights on when it gets dark only requires me to momentarily reach down and flick the switch.
I have not, at this stage, set up brake switches, but a few ideas have been considered for implementing them. The provision exists however.