John Gill Commentary Joshua 4:5

"and Joshua said unto them, Pass over before the ark of Jehovah your God into the midst of the Jordan, and take you up every man of you a stone upon his shoulder, according unto the number of the tribes of the children of Israel;" — Joshua 4:5 (ASV)

And Joshua said unto them
The twelve men:

pass you over before the ark of the Lord your God into the midst of
Jordan ;
that is, they were to go back to Jordan again, which they had passed over, and go into the midst of it, where were the priests bearing the ark; for it is not to be imagined that the ark went along with them, or followed them, they going before it; but they went where that was, just before it, from where they were to take the stones, as next directed:

and take you up every man of you a stone upon his shoulder ;
This indicates they were large stones, not ones they could carry in their hands, but ones they were obliged to take upon their shoulders:

according unto the number of the tribes of the children of Israel ;
who had that day passed over Jordan, of which these stones were to be a memorial.