There have actually always been Jews living in the area that made up the ancient Roman province of “Judaea/Iudaea” throughout the past 2,000 years.
While there is no 100% reliable numbers, I think the idea that most of the Jews were “scattered to the four winds” is a myth, at least in terms of the numbers themselves. The Jews had revolted against Roman rule a few times, the Romans had a system in place for dealing with this.
When the Bar Kokhba revolt happened, Emperor Hadrian essentially said “well it’s about time for us to get serious” and I think it highly likely some 80-90% of all Jews in the world died around that time. While nothing is conclusive, by the 1750s the world Jewish population was only estimated to be around 1.2m. If the classical figure of 550,000+ killed by Hadrian’s armies is true it is highly unlikely that many were actually left, in terms of percentages.
One of the big things that happened as a result of the Bar Kokhba revolt is a lot of Messianic Jews (those who believed Jesus was the messiah) ended up becoming Christians, and also importantly Christianity started being seen more and more as a separate thing not part of Judaism. Prior to that there were still lots of people who considered Jesus the Messiah who also considered themselves Jewish (this was a legitimate historical fact and should not be confused with the weird form of Protestant Christianity that exists in the modern era which calls itself Messianic Judaism.)
The Jews were not banned or forced out of Judaea, as some people seem to think. They were prohibited on pain of death from entering Jerusalem except for one day per year (to mourn their dead), but Jews continued to inhabit that area continuously up until the modern day. Because their population had been immensely reduced by massacre and also reduced some by people leaving Judaism for nascent Christianity it is just the case that they became a very small minority in their historical home land.
The popular mythology that the majority of the Jews packed up and left is, to me, unlikely to be true. What is probably true is that with the Jewish population nearly wiped out, some groups decided to move on. The ones that moved on to Europe where they enjoyed steadily increasing standards of living and reproductive rates numbered in the millions by the early 20th century whereas the small scattered communities in perpetually poor Judaea remained a very small population, but percentage wise I would say in the immediate years after the Bar Kokhba revolt the majority of remaining Jews actually stayed in Judaea.
There have also been persistent Christian inhabitants of the area, as well. When Muslims conquered the region eventually all the way into Greece the Christians living there did not just disappear. Most of them converted, fled, or died, but there have also always been small Christian communities in the area. It is a persistent myth that the area was totally Muslim for hundreds and hundreds of years. And when European Jews began moving into the area they were not the first Jews to have been in the area since the Romans. Some 400 years after the Bar Kokhba revolt there were still something like 50+ Jewish settlements in the area coterminous with the Roman province of Judaea.