John Gill Commentary Numbers 21:35

"So they smote him, and his sons and all his people, until there was none left him remaining: and they possessed his land." — Numbers 21:35 (ASV)

So they smote him and his sons, and all his people
They engaged in battle with him, slew him and his sons that came with him, and all his armies; and which consisted, as is probable, of all able to bear arms in all his cities; which the more easily came into the hands of the Israelites after this battle, in which such a carnage was made:

until there was none left him alive ;
so universal was the slaughter at the battle, and in the cities that fell into their hands; they utterly destroyed men, women, and children, (Deuteronomy 3:3Deuteronomy 3:6) :

and they possessed his land ;
in which were sixty cities fenced with high walls, gates, and bars, besides a great many unwalled towns; these were possessed by the half tribe of Manasseh, (Deuteronomy 3:4Deuteronomy 3:5Deuteronomy 3:13–15) .