I want to know who is eligible to give fatwa in all the schools of thoughts/fiqhs.
as far as i know in SHIA's only marja's can give fatwa.
what is the criteria in all other sects.
You have to know the detailed teachings of the Qur'an, Prophet S.A.W's Sunnah and Hadith. It is recommended that a scholar come forth with a fatwa as people not qualified in terms of authentic knowledge may only create more and more problems.
In the regard for being authentically knowledgeable, a Mufti or Alim is a preferred person to get a fatwa from. But it is advisable that a fatwa should come from a group of scholars instead of one. One person can be at fault or could make a mistake, but there is less likelihood of a complete group of scholars from making that mistake.
We only take Fatawa from Big Scholars like
May Allah have mercy on them
Other Scholars should acknowledge a scholar that he is a Big Scholar
How they will do it? By knowing his level of knowledge in Qur'an, Hadith and Fiqh.
Check this site: https://islamqa.info/en