Recursive BLE UART Broadcast? (Questions regarding Bluetooth "Mesh")

where is the code that I uploaded stored in the Puck?

RAM, unless you type save() on the left-hand side, or choose save on send in the IDE.

Cause I noticed that everything that I have uploaded seems to be lost once I take the battery out

Yes, just type save() on the left-hand side and it'll resume in the same state at power on. Create a function called onInit() first if you want something to be called at boot.

function ... Is it overwritten or appended to the end?

If you send function foo() {...} it'll overwrite any existing function with that name, or will create a new function if one didn't exist. If you send normal commands like digitalPulse then they'll be executed and then forgotten.