SSH most of the time of course, and a management interface (iDRAC, iLO, etc) if you have an enterprise server; otherwise an old monitor and spare keyboard. Some

SSH most of the time of course, and a management interface (iDRAC, iLO, etc) if you have an enterprise server; otherwise an old monitor and spare keyboard. Sometimes they'll support serial out that you can use over a cable to another computer instead of the whole monitor+keyboard combo. Or nowadays you can use a network KVM like a PiKVM, NanoKVM, or JetKVM