From the author on twitter[1] "The background music is a cello performance by Iratxe Ibaibarriaga and composed by Aleix Ramon"

[1] https://x.com/nealagarwal/status/1998788695449808920

Glad you enjoyed it! You can get the soundtrack on Bandcamp: https://aleixramon.bandcamp.com/album/size-of-life-original-...

Tomorrow it should also be on Spotify, Apple Music, etc.