The basic answer is "slow, gradual diffusion". In Late Antiquity Jews moved around quite a bit in the Roman Empire, basically all around the Mediterranean. Over Late Antiquity and the Early Middle Ages these populations diffused out, mostly north into Germany/France. We often don't know much about these Jewish communities besides their existence--we don't know precisely why they spread to different regions, or exactly when. European rulers in this era were very inconsistent when it came to their treatment of Jews, so it's possible part of the movement around was to find more friendly governments.

The next big movement was the expansion of Central European Jewish communities East, to Eastern Europe (modern-day Poland, Belarus, Lithuania, Ukraine, etc). While some scattered Jews had migrated to this area, a large number moved from Germany and France to make this a major center of Jewish population. They were in part "pushed" by Crusade-era anti-Jewish violence, but also "pulled" by a Polish government that had a relatively friendly Jewish policy. This brought brought Jewish populations East, to diffuse gradually around Eastern Europe too.

Jews during this period generally were not working in agriculture (though some were), which contributed to this. A population economically tied to its land can't just "diffuse" over time so easily, but people working along trade routes will have an easier time gradually moving around.