The {S} symbol is a generic mana symbol. It represents a cost that can be paid by one mana that was produced by a snow source. That mana can be any color or colorless.

Snow isn’t a type of mana. If an effect says you may spend mana as though it were any type, you can’t pay for {S} using mana that wasn’t produced by a snow source.

In most cases, the land that becomes a creature will remain colorless. If another effect caused the land to have one or more colors, the resulting creature land will retain those colors.

A land that becomes a creature because of Avalanche Caller’s activated ability will retain any other supertypes, card types, subtypes, and abilities it had. In particular, it will be a snow creature land.

Some cards have additional effects for each {S} spent to cast them. You can cast these spells even if you don’t spend any snow mana to cast them; their additional effects simply won’t do anything.

The Kaldheim set doesn’t have any cards with mana costs that include {S}, but some previous sets do. If an effect says such a spell costs {1} less to cast, that reduction doesn’t apply to any {S} costs. This is also true for activated abilities that include {S} in their activation costs and effects that reduce those costs.

Snow is a supertype, not a card type. It has no rules meaning or function by itself, but spells and abilities may refer to it.

The activated ability doesn’t cause the target snow land to become tapped or untapped.