what do historians rely on for their study
Answers
Africanists tend to rely on matrices of oral history for two [main types of] cases.
First, if a society is nonliterate (which is different from "illiteracy," in that it's not a case of lack but a case of distinction), we look for epics, usually poetic or performed, that have come across time and name people as well as events and locales. Nonliterate societies tended to have mechanisms for transferring this knowledge that were fairly robust: unlike the telephone game we all probably played in grade school, this was an entire caste moving knowledge from generation to generation. It would still change, especially in response to the needs of the present for the knowledge, but those additions and idiosyncrasies can be found and often eclecticism was limited by it. The signal example of this is the story of Sundiata Keita, the first great mansa of a united Mali. A number of transliterations exist including one in film, each marked by the circumstances of its collection and its purpose, but virtually all run through the Kouyaté clan of djeli or griots. Even then, most of the oral history deals with the political elite and the privileged; sometimes you'll get at important caste histories in that way too (for ironworkers, perhaps). But the majority of people mostly remain left out, much as they do with written historical records.
So in cases where a society has been connected to literacy, oral history is also a way of getting at the recent past for the many people left out of the written record. Sometimes this operates as the collection of oral testimony in the field, and other times as family histories or tales that elucidate something left unelaborated in a written record. Subaltern studies deals with a lot of this kind of knowledge and the pitfalls of ever fairly representing the speaker, and it's not at all unique to Africa. But it can add a tremendous amount of knowledge about the context of state documents, which have their own positional issues.
In both cases, the strength of the oral evidence is greatly enhanced with corroboration by written documents, archaeological data, or other kinds of additional support. But that's true of almost any historical matter, even those in writing