It was conveyed from the Prophet salla Allahu alaihi wa sallam to the companions to the narrators one by one. The scholars have scrutinized each and every man in such chain of narrators and studied their lives to determine whether they are trust worthy of narrating the hadeeth of the Prophet salla Allahu alaihi wa sallam or not. You have to bear in mind that Allah has pledged to protect the Quran and the Sunnah.