They would shut down every single process on the server and bind the trading trading app to the CPUs during trading hours to ensure nothing interrupted.

Electrons travel slower than light so they would rent server space at the exchange so they had direct access to the exchange network and didn't have to transverse miles of cables to send their orders.

They would multicast their traffic and there were separate systems to receive the multicast, log packets, and write orders to to databases. There were redundant trading servers that would monitor the multicast traffic so that if they had to take over they would know all of the open positions and orders.

They did all of their testing against simulators - never against live data or even the exchange test systems. They had a petabyte of exchange data they could play back to verify their code worked and to see if tweaks to the algorithm yielding better or worse trading decisions over time.

A solid understanding of the underlying hardware was required, you would make sure network interfaces were arranged in a way they wouldn't cause contention on the PCI bus. You usually had separate interfaces for market data and orders.

All changes were done after exchange hours once trades had been submitted to the back office. The IT department was responsible for reimbursing traders for any losses caused by IT activity - there were shady traders who would look for IT problems and bank them up so they could blame a bad trade on them at some future time.