The Rito are sure to have a rough go when you first meet them in The Legend of Zelda: Tears of the Kingdom. They are being tormented by a giant blizzard that has completely stalled any trade or income they need to survive.
As you set out to help the Rito population, you will encounter a few who are in need to help with a situation that they can't handle themselves. You will complete the quest Crossing the Cold Pool as a way of helping out a specific Rito, should you encounter them in the snowy wilderness.
Start The Crossing The Cold Pool Quest
If you are looking to complete the main quest, Tulin of Rito Village, then you will come to a part where you are asked to look for the strong-willed Tulin as he has gone off to scavenge for food. You will work your way up a snowy slope, encountering a few different Rito who you can ask for Tulin's location.
At the top of the slope is a Rito standing in front of a cave named Verla. If you speak with him, he will tell you that he needs to get 10 Chillshroom for the village, but because of his injured wing, he can't fly over a small pond of freezing cold water to get them. Now, it's up to you to help him get those Chillshrooms.
Complete Crossing The Cold Pool
If you are fortunate enough to already have 10 Chillshrooms in your inventory, then you can just hand them over right now. However, if you don't have them already, or you just want to complete the quest as a whole, then you will need to do some building.
If you go into the cave, you will see the pond that Verla is talking about, and he is not kidding about it being freezing cold. If you hop in, you will quickly start losing health and die. So, don't even bother trying to swim to the raft at the other end of the pond.
Instead, go back outside, and you will notice a bunch of wooden platforms that you can attach together to make a bridge. The best thing to do is to take the two longest pieces from the construction site and attach them end-to-end. Then, take another smaller piece and place this on one end of the bridge you are making.
You can now pick this up with Ultrahand and carry it into the cave, placing it on the pond to give you a way to cross it. You will have to do a small jump to get onto the raft since the bridge won't extend all the way across. But you can then get onto the other side of the cave from the raft and begin picking all the Chillshrooms here.
You will eventually have 10 Chillshrooms in your inventory and can take these back to Verla. Once you give them to him, he will thank you for your help, and the quest will come to an end.