Your historical knowledge is completely wrong. First Janissaries in Murat 1 reign (1363-1389) was captives and Sultan took 1/5 captives of Akinci’s from raids and trained these captives as soldier. Ottoman raided all Balkans (Serbs, Bulgarians, Wallachians, partly Hungarians) not only Byzantine lands. In Murat 1 reign, Byzantine lands got shrink a lot by Ottomans. Only remaining land of Byzantine Empire was Constantinople and its surroundings and Byzantine Empire was obliged to send tribute and soldiers to the Ottoman Empire. This means that Byzantine Empire became vassal state of Ottoman Beylik. I share this map in order to show Byzantines borders. Thessaloniki is conquered in 1385 but it was retaken by Byzantines 2 times in 1389 and 1402 during Ottoman throne is empty. Ottoman lastly retook Thessaloniki in 1430 from Venetians. It was intense Siege, it cost the Ottomans heavy losses but ultimately, Ottomans prevailed over Venetians.
Anyway In Bayezid 1 reign (1839-1402), Janissaries become famous devshirmes. They are taken from Christian (not from Jews) population (Balkan races, Greeks, Armenians, Georgians, even some Christian Turks (remnant of Turk mercenaries from Byzantines)) of Ottoman Empire. In Mehmet 2 the Conqueror reign (1451-1481) all Janissaries (Contrary to general belief, their number was around 6.000 (10.000 number isn’t reliable)). In 1568, their number increased to 12.789, in 1609 37.627, in 1670 53.849) is equipped by matchlock. They still used bows until 18th century and melee weapons (spears, swords, shields, axes) until 19th century but matchlock became their main weapon after 1450s. Muskets replaced matchlock in the 16th century.