Why is my PC typing and triggering shortcuts with the keyboard unplugged?

Start with the other input devices. A wireless keyboard can have its own USB receiver even when Bluetooth is off, and a mouse or other USB device could be involved too.

Disconnect peripherals one at a time, including any wireless receivers, and see whether the behavior stops. If it does, reconnect them individually to narrow down which one is involved.

If this is a laptop, unplugging an external keyboard leaves the built-in one connected. A fault there could still produce input. On a desktop, check for another keyboard or input device you might have forgotten was connected.

Take a look at the keyboard and accessibility settings, including Sticky Keys. You could also try reinstalling the keyboard driver. Those are troubleshooting steps rather than a definite fix, especially since the physical keyboard is unplugged.

A clean malware scan doesn't prove the machine is clean, but random shortcuts alone don't establish malware either. The fact that it continues offline and without the keyboard makes checking other input devices and software settings a sensible next step.