How did al-Bukhari and Muslim collect the Hadith?

Imams al-Bukhari and Muslim collected the hadiths in their respective books from both oral sources (from their shaykhs) and written and compiled sources. These written sources include the hadith books that appeared in the early 2nd century AH, such as the "Musnads" and books arranged by jurisprudential chapters, which in turn contained books and scrolls from the 1st century AH, like "Al-Jami‘" by Abd al-Malik ibn Jurayj and "Al-Muwatta’" by Malik ibn Anas. Dr. Fuat Sezgin emphasized that al-Bukhari aimed to compile a concise, authentic collection from the works of the two preceding generations, and he did not only benefit from hadith books but also utilized linguistic, historical, and jurisprudential texts. It is believed that al-Bukhari and others did not collect their hadiths solely from oral sources but relied on documented books. The mid-2nd century AH witnessed the emergence of major hadith encyclopedias, where scholars depended on previous books and writings. Al-Bukhari and Muslim combined the efforts of those who preceded them, selecting authentic hadiths through narrations with chains of transmission tracing back to the authors of numerous books and directly from shaykhs.