Did you skim through it? It's mentioned in Ch 2 https://www.mangago.zone/read-manga/please_love_me_upi/uu/iur_chapter-2/pg-4/.
The hyung's personality isn't well tackled in the story but the author gave a break down of their personality here. Turns out they imprinted each other and the hyung is the first to fall. https://www.mangago.zone/read-manga/please_love_me_upi/uu/br_chapter-55032/pg-1/
In the extras it explains that seme is a hawk anthromorph and uke is a falcon even though you dont see his wings. The anthromorph thing was mentioned briefly in the story
It looks like the storyline just appeared as it's being drawn. There's no background like What's his hyung is thinking or feeling? Why do they have wings? Do every other have wings too? The author just served it like I've wrote this years ago have it dince it's better than rotting in my drafts