Pico-unicorn hat and additional sensors

I’ve got Pico and pico-unicorn hat and I’d like to add PIR sensor to the setup. Which pins I can connect to
(which ones are not in use by)?

Also in terms of best practice how? Soldering the PIR first and then how to attach hat, so that it’s actually connected?

The pinout for the Unicorn Pack is on its shop page.

One way to do it would be to put a Pico Proto between the PICO and the Unicorn Pack.
Pico Proto – Pimoroni
Using stacking headers.
Pico Stacking Header Pack – Pimoroni
Run wires from the Proto to the PIR