Because Blessing of Frost doesn’t target any creatures, you determine how the +1/+1 counters will be distributed as Blessing of Frost resolves. Players may respond to Blessing of Frost as normal, and they’ll know how much snow mana you spent to cast it, but they won’t necessarily know how the counters will be distributed.

Because none of the creatures are targeted, you can choose to put +1/+1 counters on a creature with protection from green, for example.

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.

If you cast Blessing of Frost without spending snow mana, no +1/+1 counters will be distributed. You may still draw cards if your creatures are large enough.

If Blessing of Frost is copied, no mana was spent to cast the copy, so no +1/+1 counters will be distributed. You may still draw cards if your creatures are large enough.

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.

You determine how many creatures you control with power 4 or greater as Blessing of Frost is resolving, after +1/+1 counters have been put on creatures.

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.