Virtually all mainstream secular scholars of early Christianity agree that Jesus existed. He was mentioned by Josephus and Tacitus, and the Talmud - a religious commentary, albeit not a Christian one, with genuine historical narrative interspersed - mentions him repeatedly. Essentially, based on the sources we have access to today, it’s far more likely that Jesus was a real wandering preacher in the first-century Land of Israel (there were many of those!) than that every mention of him in the sources was fabricated.
My understanding is that scholars also believe that Siddhartha Gautama, the historical Buddha, was indeed historical, but the contemporary sources are weaker.
> My understanding is that scholars also believe that Siddhartha Gautama, the historical Buddha, was indeed historical, but the contemporary sources are weaker.
This is correct and as for which parts of the text are original, philologists such as KR Norman, Gombrich, Vetter and Bronkhorst have come to some consensus (like on the four noble truths and dhyana), excluded some things and some things are still debated.
My understanding is that scholars also believe that Siddhartha Gautama, the historical Buddha, was indeed historical, but the contemporary sources are weaker.