When and how did Bulgaria and Serbia come under the rule of the Ottoman Turks?

The period of their conquest lasted from the mid-14th to the mid-15th century. First, the Turks intervened in the internal conflict in Byzantium and in 1351 captured the first fortress in the Balkans, from where they led the expansion to the west. In 1389, they had already defeated the Serbs in the Kosovo field (but they were able to kill the Sultan), and by 1422 – the Vidin kingdom in Bulgaria. The success was ensured by the numerical superiority and conflicts in the Balkans, even foreign aid did not help against the Turks, in 1396 and 1444 two crusades were defeated near Nikopol and Varna.

They conquered Serbia and Bosnia after the capture of Constantinople, by 1463, although Herzegovina and Zeta held out until 1482 and 1499. They took Belgrade around 1520, but it was a Hungarian fortress.