I'm American and YOASOBI is one of my favorite artists right now :) I'm always astounded by Ikura's voice. And even though I need translations, I love the stories and music videos intertwined with the music. I just think it's such a unique sound and concept.
I think you can explain 青春 as “the passion of youth.” It’s a feeling of excitement and novelty at the world around you that you’re experiencing for the first time. In your teenage years, you’re slowly entering the world of adults and opening your eyes to new experiences. Taking up a hobby, for example, seems so important and exciting when you’re a teenager but when you’re older the same activity doesn’t give you the same feelings. You have other things in your mind, like work, bills, and daily life. So, that activity doesn’t absorb you the way it did when you were a teenager. 😅