I am from Bangladesh, and we do lots of rituals when burying the dead like saying 'La ilaha illa Allah' and other stuffs. Is it from the Sunnah?

No this is not according to Sunnah. When people die, what the Prophet Muhammad (ﷺ) did, what his companions did for those who die are well known acts. Making du'a for the dead, giving charity, these are the known things. But gathering on so many days, seventh day, fortieth day, these kinds of gatherings or doing things like specifying amounts of dhikr to do or reading of Qur'an and giving the reward of it to the dead, this is not from the Sunnah of the Prophet Muhammad (ﷺ).