John Gill Commentary Joshua 18:20

"And the Jordan was the border of it on the east quarter. This was the inheritance of the children of Benjamin, by the borders thereof round about, according to their families." — Joshua 18:20 (ASV)

And Jordan was the border of it on the east side
It had Jordan on the east, Dan on the west, Judah on the south, and Joseph or Ephraim on the north:

this [was] the inheritance of the children of Benjamin, by the coasts
thereof round about, according to their families ;
this is the general description of the limits of this tribe, the particular cities in it follow.