Switchbot similar idea

Hello everyone,

I want to be able to open my building’s main door wirelessly, so I was going to add an arduino with a relay in my intercom system, and the relay would “press” the button to open the door. My problem is that I have no way to power the system, so I was looking for a solution for this. I know this is doable as the Switchbot can work for 1/2 years on a battery and the principle is similar, the thing is I am not sure which sensor to use (wifi or bluetooth). I think Bluetooth is better, but it would have to constantly listen to a signal, and I am not sure how to do this without drawing too much power. Btw, using a Switchbot to press the physical button in the intercom is also a no-go. Hope you can help me.

Thank you in advance for your help.