They did that smartly.
The rest of WWII continued on for 5 years or thereabouts.
The lesson for everyone ~ the Finland had an army that could beat both the Russians and the Germans. The rest of the world took 5 more years just to beat the Germans.
Irrespect of how folks want to make the war between Russia and Finland look, they must also consider the war between Germany and Finland.
Finland was no one's co-belligerant. On the other hand, in the German war against Poland and the Russian war against Poland, Germany and Russia were co-belligerants.
OK, whatever you want to call it, Finland was fighting against the Russians in the Leningrad theater at the same time that the Nazis were besieging Leningrad, into 1944, and General Mannerheim accepted the Iron Cross sent to him by Hitler for Finland's participation in the fighting. It is true that the Finns did not advance beyond the pre Winter War border or fire artillery into Leningrad, both of which the Nazis wanted them to do.
By the way, my father was a 100% Finnish American, and I always insist that Finland be labeled as a co-belligerant, not an ally of Germany, as so many mistakenly refer to it.
I don’t think it made much sense for the Germans to stay in Finland after the Finns capitulated. I read a recent history of the 11th SS Nordland Division (Twilight of the Gods) by a Swedish SS volunteer. Although there was some scrapping along the way the Germans essentially withdrew to the ports and went home.
The Finns were forced to capitulate and weren’t too keen on attacking their former allies. The Russians, of course, insisted on full scale attacks by the Finns on the Germans. I don’t think the Finns did any more than they had to do in attacking the Germans based on the authors story.