Connect to mdb42q from pi and control it ?

I think @fanoush has provided loads of info already.

But there is also https://www.espruino.com/Interfacing#bluetooth-le which provides you a bunch of examples to turn the LED on and off using Web Bluetooth, Node.js, Python, even the Bash shell - all of which should work on the Pi.

For instance this single command when run in the shell on Raspberry Pi should do it once you use the Mac address of your MDBT42Q:

gatttool --addr-type=random --device=F5:3E:A0:62:C7:74 --char-write-req --handle=0x0010 \
 --value=`echo "LED.toggle()\n" | od -A n -t x1 | tr -d " "`

Worth noting that a few of the examples use LED.write, LED.set, LED.toggle - but as @fanoush noted above they are all just different ways of controlling a pin: https://www.espruino.com/Reference#Pin