I only about Simo Hayha who killed two soviet divisions.
Finland didn't win. They lost both of their wars against the Soviet Union (the Winter War and the Continuation War). When the Soviet Union began the Winter War, they set up a puppet government, the Finnish Democratic Republic (the Kuusinen government, in Terijoki), and apparently intended to quickly conquer Finland on behalf of this government. When the expected-to-be-a-few-weeks war dragged on for months, with a growing prospect of British and French intervention which would at least drag Norway and Sweden into the war, the Soviet Union wanted out ASAP. Once they won battlefield victories which (a) restored the prestige of the Red Army, at least in their own eyes, and (b) forced Finland to accept a peace deal they weren't happy with (but was better than continued war and conquest), a peace was agreed. See my answer in
for more on why the Soviets were in a hurry to finish the Winter War (and also start it in the first place).
The casualties in the Winter War were quite lopsided. Finnish casualties were about 70,000 (of which about 26,000 were killed), and Soviet casualties were probably about 350,000, 5 times higher than the Finnish casualties (estimates of the Soviet casualties vary from about 300,000 to over 1,000,000, but the high estimates are very unlikely to be correct). However, having a favourable ratio of casualties is not the same as winning the war. Similarly, in the Continuation War, the ratio of casualties favoured Finland, about 225,000 vs about 900,000, but the Soviet Union still won.
As for the Continuation War, the second part of Finland's WWII, Finland halted the final offensives by the Soviet Union at considerable cost to the Soviets (and also to the Finnish forces involved), and the Soviets had some very important distractions, like trying to defeat Germany as quickly as possible. In the final large battles, the Soviets had at least 5 divisions wrecked:
and while they had plenty more to commit to Finland, Germany was the important enemy. They were prepared to accept peace with an independent Finland, and offered and got it. For more, see the answer by u/Holokyn-kolokyn
It isn't known for certain what the Soviet goals at the end of the Continuation War were. Many suspect that their goal was conquest and the imposition of a puppet government, possibly to be followed by absorption into the Soviet Union (as Estonia, Latvia, and Lithuania were). Others claim that the Soviet Union only wanted to force a peace deal like the actual peace deal on Finland. This is discussed by: