Start with a IR receiver for < $1. Add a $2.50 RP2040 zero to any sensor, and a few lines of code with some AI help, or not, and you have a HID (human interface device) that plugs into your USB port. It's natively recognized as a mouse, a keyboard, or even a midi device. This gives you more control than a keyboard or a mouse however. 

Sensors or other input devices can have very powerful capabilities. An IR receiver, an RFID card, a fingerprint reader, a magnetic reed switch, a proximity or "human" sensor (pick your flavor), a BLE device, a WIFI device, an App, or whatever "controller" or sensor you want to attach. 

Do you want your computer to take some action when you do a secret dance?  
You can do that!  But please don't. 

Using Wifi, or other protocols that can send information to the microcontroller, you can continuously send data, or commands to the computer. I'm not a hacker, so I don't endorse taking data back out of the computer, but of course that's possible as well. It's really hard to stop someone with physical access to a computer from accessing it in a way that is convenient, or practical, if they know what they are doing. 

Have fun!