What is this square hole for?
Cabin pressurization outflow valve. 

Cabin pressure in an airliner is maintained by bleeding air from the engine compressors into the cabin, after conditioning (usually cooling and humidifying) it. Current designs are beginning to use separate air compressors for this. 

In either case, the cabin pressure is regulated by controlling the difference between inflow, provided by the bleed or compressor, and outflow, which is a simple motor-controlled valve. 

Generally the inflow is set to maximum and the outflow is varied, as this provides the maximum fresh air flow for the passengers. 

See also:

How does the outflow valve work on a cabin pressurization system?

How does the cabin pressure controller work in a large commercial airliner?

How does the cabin pressure controller work in a large commercial airliner?

https://www.airspacemag.com/flight-today/how-things-work-cabin-pressure-2870604/

Can an aircraft re-pressurize at high altitude?