How do Historians Learn about the Past?

Have you ever thought how historians come to a theory? Do they just guess? No they don't, historians use things called sources, sources are things that give evidence to a certain thing. Historians first have to find these sources through, excavating or other ways. But what do they do after they find the sources? They put them in two categories; written and non-written. Written sources can be; diaries, books, newspapers, ads, poems and others. Non-written sources are like artifacts, example; pots, paintings etc. But what happens after that? The historians look for three thing; validity, reliability and date of origin. Date of origin mean when it as made, reliability means how reliable is it, example; who is it made by etc, and validity; is it impartial or to biased.