Yamal and Gavi scored as Barca won 2-0 in Jeddah on Wednesday night.
"The goal was to reach the final and we are very happy with the victory," said Yamal afterwards.
"Athletic is a very physical team, which makes you run a lot. We suffered at the end of the first half but we knew how to play and we are very happy.
"Whenever the manager gives me confidence to play, I am very happy and glad to be able to help the team."
On facing Real Madrid or Real Mallorca in the final, Yamal said: "We don't care who we're going to play against in the final. We hope to win it and go home with the title."
On the news of Dani Olmi and Pau Victor being temporarily registered after a successful appeal, Yamal added: "We are very happy that Olmo and Pau Víctor can now play. I hope they can play in the final.
"It is normal to be nervous when you cannot play and when you get news like that, you are very happy. And we are very happy for them."