Any DIY replacement ideas ? I have spare electronics
It's low-voltage. The wires you can see would never be rated for mains voltage. I doubt that anyone, anywhere has ever made a doorbell that runs directly on mains voltage. It would be far too dangerous.
The bell itself doesn't care about AC vs DC, so probably AC for simplicity.
As far as replacement, either get another one just like it, or build a small circuit that rectifies/filters the AC and apply the resulting DC to any electronic beeper or sound generator you like.
The fact that it's low-voltage does not mean it is 12 volts. 18V is commonly used because it's a former quasi-standard for doorbells, but 24V is much in vogue because it can be easily poached off the thermostat 24V, which is a standard.
Older installations are almost certainly AC. Some bells care about the difference, others do not. Plain switches don't care about AC vs DC, but that statement only applies if voltage is below about 30V.
Just measure across the open switch. Or across the bell while dinging.