You will need data before you need anything else: for the bus ticket, the map to the apartment and the DVV booking. Two ways to have it on the way out of the airport.
Option 1: travel eSIM, activated before you fly
If your phone supports eSIM, buy a Finland or Europe data plan from a travel eSIM provider, install it at home, and switch it on when you land. A week or two of data costs a few euros to twenty. It has no Finnish number, which does not matter for the first days.
Option 2: Finnish prepaid at the airport
At the R-kioski in the arrivals hall, buy a prepaid starter pack from Elisa (Saunalahti), DNA or Telia. No ID needed. You get a Finnish number, some credit and the option to buy a data package. Prepaid data costs more per gigabyte than a contract, but you can keep the number when you later move to a contract at the same operator.
Then: a contract
Once you have a henkilötunnus and an address, move to a monthly contract; unlimited data for a resident costs a fraction of prepaid packages. The comparison is on the mobile plans page and the explanation of the operators in mobile plans explained.