Industrial PCs are also commonly passively cooled and more or less sealed, using the case itself as a heatsink and protecting the electronics inside from airborne contaminants.

Individual components can also be industrial or automotive grade rather than commercial grade; for example, industrial-grade SSDs are usually rated for 85°C and often have much higher write endurance ratings than client/consumer SSDs, and may also be protected with a conformal coating or epoxy potting across the entire board.

This is correct. The hardware is built and tested to be run in the most horrible conditions. Their support is often top notch too for when the equipment breaks down.

Think of them like "rugged laptops" but for PCs / servers.

They also tend to have lots of legacy IO and expansion slots. Like 8 bit ISA slots on modern hardware.