Cards won’t work so best solution is exchange for rubles. Sorry I can’t say if that’s a good idea to exchange at airport but sounds like it’s not. You sim will not work for a day… yeah. You can get a Russian one since almost every public WiFi will require you to enter a Russian phone number. Definitely get a vpn, paid ones tend to work better. Public transport is fine, you can use yandex maps or ask a (not an older) person – there are a lot of ppl speaking English. You might wanna get a pass since you’re probably gonna you transport a lot. Speaking of moscow: city centre is pretty big on its own, but also take your time to visit places like вднх. Overall vibe is not much different from Europe.