Why does the chromaticity diagram look like that?

I’ve been having problems studying this topic for years now, is there actually an official scientific field with official books and an official consensus on this ? Seems hard to know who to trust on this wide-but-niche topic

Yes. See, e.g.,

https://en.wikipedia.org/wiki/Color_science

https://en.wikipedia.org/wiki/Colorimetry

https://en.wikipedia.org/wiki/Photometry_(optics)