Chain Of Hadeeth
The experts knew these people. They recorded their biodata in their respective times which are consulted by others.
السلام عليكم As you know, terminologically, these are called اخبار by scholars. Raw, these are unreliable and that is why work on them began in the first place: because people were ascribing things to the Prophet صلى الله عليه وسلم which he never could have said. However, the Muhaddithoon soon were able to come up with criteria for testing their validity. Even though they were not able to always follow these criteria, as they like all of us were human, we still have the criteria itself. So, as long as we honestly weigh these reports on the scale the Muhaddithoon created, we can use them to discover the life of our Prophet صلى الله عليه وسلم and his companions رضي الله عنهم اجمعين. However, it should be kept in mind that these are not a source of religion. We, the general populace, and now the scholars too, has often confused these reports with the Sunnah, which, along with the Quran, is the only source of God-given religion. Scholars have written on this, for example Allama Tamanna Imadi in his “book,” کوئی ہے جو اس سوال کا جواب دے؟.
I am not sure if this has already been discussed or not. Even if people are dead in every chain, if multiple independent chains lead to the source and all independent chains report nearly the same information, then the info is likely to be reliable. Still, Hadith should be read for informational purposes only. The entire deen is in Quran and Sunnah.