How do capacitive pens work?

You electrically couple to the pen, unless they are active devices, in which case they generate a signal.

That can't be, they work with gloves and things. My gloves even have a thing at the end that works with the touchscreen.

You’ll have to be more descriptive for me to have any hope of answering.

If by your gloves have a thing at the end you mean a patch of special fabric, yes, that fabric conducts from your skin to the screen.

Tape a wire to the trackpad and hold the wire?

ground mass?

If you don't have ground human presumably you can substitute ground beef or ground turkey.