It's true that Birds of Feather feels "unreal", not genuine, and more like a contrived pretense. Like much of Billiy Eilish's whole persona. However, in essence that is what all "love talk" is really pretense. When someone is really in love it feels more genuine, on account of their delusion being believed by them as well, but you can enjoy the song on the basis that most talk of love is not genuine anyway.
it's a fair criticism though, clearly at her age she would not have loved in that way yet, it is an artistic piece after all, not a realistic one. I certainly agree the song feels somehwat unreal, not genuine.
She does write her own lyrics though, with her brother Finneas. They're both quite creative and original.
I would say her voice is certainly quite pleasant, even if it lacks the distinctiveness of Lennon, Starr and McCartney, who actually were also pretending to be Americans most of their lives. Where she really shines is in the harmonies, the belting, her singing is rather more accomplished and ambitious than Lennon and McCartney, though obviously less intense.
Clearly the percussion is produced electronically, and real instruments are always preferable, but in this song the elecronic music is produced well and fits the song perfectly. It's rather the opposite, this song shows that even with modern instrumentalisation you can still produce good music.
She does cultivate the creepy vibe a bit, but she was not born a boy, come on...