1) When I am praying behind a Sunni imam, am I doing so with the intention of Jammat or individual? Because I read in Sistani’s Book of Islam Laws – “Ruling 1433. The imam of congregational prayers must be bāligh, sane (ʿāqil), a *Twelver Shia*, just (ʿādil), of legitimate birth, and a person who performs prayers correctly”

2) If I am praying with the intention of Jammat, do I read the Fatiha and the Surahs in Fajr, Maghrib and Isha loudly or quietly?

3) If I am praying with the intention of Jammat, will their Jumuah congregational prayer suffice for Zohr?

by

1- You just follow him in acts, but recite Quran for your self.
2- No need to recite loudly.
3- If you recite Fatiha and sura for yourself, then yes