Mates
Start simple
Coffee or a walk for a couple of hours is a good first step: short, cheap and in a public place. One or two meetups a week and you already see familiar faces.
Meetups in your language and in Polish
It is easier in your own language at first: Mates has meetups in English, Russian and Ukrainian. Once you settle in, try meetups in Polish: good practice and new friends among locals.
Stay in touch
People from shared meetups stay in “My people”. Add them as friends, message them and invite them to the next meetup.