Nothing wrong with researching the underlying docs.
The internal pullups are "weak", meaning the device connected needs to be able to sink
5V/20,000 ohm = 0.25mA, which is next to nothing electrically. The resistor is enough to hold the pin high when nothing is connected.
A mechanical switch connecting the pin to ground is one of the most common things done.
Have your code act when it see the pin going low.
Add a TVS from the pin to ground for static electricity protection.