John Gill Commentary Joshua 18:16

"and the border went down to the uttermost part of the mountain that lieth before the valley of the son of Hinnom, which is in the vale of Rephaim northward; and it went down to the valley of Hinnom, to the side of the Jebusite southward, and went down to En-rogel;" — Joshua 18:16 (ASV)

And the border came down
In the description of the border of Judah, hereabout, it is said to go up, (Joshua 15:5) ; because there, as Jarchi observes, the measure was from east to west, but here from west to east:

to the end of the mountain that [lieth] before the valley of the son
of Hinnom ;
this south border of Benjamin is the same with the north border of Judah; and the same places are mentioned in the description of the one as of the other, see (Joshua 15:8) . The mountain is Mount Moriah,

[and] which [is] in the valley of the giants on the north ;
on the north of the valley of Rephaim:

and descended to the valley of Hinnom ;
the border from the end of Mount Moriah to that valley:

to the side of Jebusi on the south ;
to the south side of Jerusalem, having that city on the south;

and descended to Enrogel; of which (See Gill on Joshua 15:7).