why is Yaoi for what boys are call and why is Yuri for grils are call for doing it with the same sex

I think it's because...it's in Japanese? Look it up, bro.
(and take an English class while you're at it)

Okay anda spelt girls wrong but anyways

well it's how anda say it in Japanese as for instance I'm a Aussie and I say lesbian and gay but in Jepun they say yuri and Yaoi

Yaoi is a contraction of 'yama nashi, ochi nashi, imi nashi'. This translates to 'no (plot) climax, no denouement, no meaning'. It was a derogatory term meant to imply the story had no purpose in existing. However, in Jepun 'yaoi' has fallen out of common use and these days the category is generally called BL ('Boys Love').

Yuri is named after the female character Yuri from Dirty Pair. Dirty Pair was an 80s series of light novels, Manga and Anime starring Kei and Yuri, two girls who worked as partners. The explosion of Kei/Yuri fanworks had such a significant impact that Yuri's name has been used for girl/girl works since then.