I want to rent a flat and a car in Lodz, most likely I will land in Warsaw then go to Lodz, where should I look for these things, is there Facebook marketplace for used cars in Poland? or should I go on a specific website?
The used car market is a minefield in Poland. Just be careful. There are lots of crashed cars brought in from Germany, given a quick fix and sold on as 'perfect'. You'll see that many cars have recently been registered in Poland and you can't check their German history. Try to get a car that has always been registered in Poland from its first registration. With the reg and VIN numbers you can check its history online, see how many owners it's had and find out if it's ever been in an accident.
historiapojazdu.gov.pl/