I thought it was mostly just the frozen sea- and that after it thawed and just before the armistice the soviets landed some troops west of the mannerheim line ?
No, they walked across the ice.
Winters throughout the 1930s had been relatively tame. The winter of 1939-40 was the first really cold one in years, indeed it was one of the coldest winters of the 20th century.
Image courtesy of https://www.winterwar.com/other/weather.htm
Indeed at one point in January 1940 the temperature dropped below -40 °C as far south as the Karelian Isthmus, visible here as the 16th-20th January low (note that the table measures 5-day averages).
Before the war Finnish war planners hadn't expected the ice of the Bay of Viipuri to be strong enough to carry not just men, but tanks, across to the other side of the bay as late as March. Consequently the Finns hadn't fortified the western side of the bay. Not that there would've really been funds to spare for that anyway, with the military's funding being what it was throughout most of the 1930s. But in March 1940 the ice was just that: strong enough.
The situation as of 13 March 1940. "Mt." here stands for "motorised", e.g. 567. Mt.JR = 567. Motorised Infantry Regiment.
Now, by the end of the war on 13 March the ice had already began to melt, and there's a chance that had the war continued, the Russians on their beachhead north of the bay would've gotten trapped between the Finns and the thawing sea. Probably the likelier scenario is that the Finns would've had to have pulled back in the far south of the front to the
Kymi river 100 km west of Viipuri and regroup and reinforce during the spring mud season, but this is speculation and outside the scope of this thread.
At any rate, while the frozen Gulf of Finland might've hindered major landings across longer distances, it didn't stop the Russians from trying to land, or rather march, ashore near Kotka and Virolahti, some 60-100 km west of Viipuri, with a force of some seven ski battalions, using nearby islands in the Gulf of Finland as bases. They were repulsed by Finnish coastal jägers and coastal batteries.
The map uses Finnish military map symbols. One bar here is a platoon, two a company and three a battalion. Four, absent here, would be a regiment. The empty rectangles represent infantry, the ones with a circle coastal jägers, and the inverted triangles coastal artillery.