How does public transport in Warsaw work?

Hello guys. We're visiting Warsaw for the weekend and I can't seem to understand how buying tickets for public transport works.

Can you just pay by card when in the vehicle or do you need a special app?

I found some sites offering tickets to book but wasn't sure if that's the way.

We're arriving at Molin Airport and need to fork out how to get to our Air BnB and also just travel around the city for a couple of days.