The fire pump should be tied into the fire alarm system so they know if the pump isn’t working or the power is out. Regardless, if power goes out, city water isn’t working (water main break, whatever), a fire pumper truck will hook up to the FDC (fire department connection) and pump water into the system.
That’s the entire point of the FDC, it’s a fire hose connection looking thing on the outside of buildings so fire trucks can bring water into the system or add to the pressure, etc. Sometimes it’s as easy as running a hose from a a fire hydrant on the street to the FDC. This is the backup plan if the normal building water supply isn’t working.