He wasn’t alone at the top of one instant big army. He joined a lord who was already making his own kingdom, like several others, as the Qin were losing control of a lot of land. Liu Bang did become one of that lesser lord’s two top generals, and the other general, Xiang Yu, is the one who raced to the Qin capital and burned the palace.

Then Xiang Yu ordered the official split of the Qin empire into like a dozen smaller kingdoms, giving some of them to his friends, and then went around slaughtering them to reunite the land under his control, because that was his fucking jam (that was after having made his own former lord the new emperor, and then murdered him). Liu Bang was one of them, getting his own army, and ran around uniting people like smart tacticians and leaders with their own armies faster than Xiang Yu could kill them. He won that race, Xiang Yu got himself cornered and alone (and was so metal he cut his own head off in the middle of the battlefield when he was the last one standing on his side), and Liu Bang founded his dynasty with all the people who put him there.