We too are travelling through here in a few weeks time-but arriving by train from Boden which will be a 30 minute walk to the bus stop in Tornio to go to Kemi according to Google Maps. There are a few buses a day from the train station to Haparanda bus station which I’m hoping meet the train arrivals from Boden. A detour to IKEA for meatballs will be hard to resist at that point!
I see that there are buses between Tornio and Kemi that are free for Interrail pass holders.
https://benefitsportal.eurail.com/benefits/bus-services-in-finland/
And following the link on that page took me to the timetables and maps of bus stops for the bus to Kemi.