This is typically a FM200 system in the datacenter [1] which is a pressurized gas. There are also often water "dry-pipes" in place that can pressurize if the FM200 fails. Fire suppression codes vary by country.

[1] - https://en.wikipedia.org/wiki/Automatic_fire_suppression

There are gas based fire suppression system such as FM-200 and Novec 1230.

If you aren't using gas based fire suppression systems it is basically an amateur operation. The small data centre at my work (10-ish racks) has a FM-200 fire suppression system, it really isn't that expensive to set up.

Most Fir suppression systems used in Electronics are Halon, CO2, or some other O2 replacing substance