John Gill Commentary 1 Kings 6:16

"And he built twenty cubits on the hinder part of the house with boards of cedar from the floor unto the walls [of the ceiling]: he built [them] for it within, for an oracle, even for the most holy place." — 1 Kings 6:16 (ASV)

And he built twenty cubits on the sides of the house
At the end or extremity of it, as the Targum; that is, he built the most holy place, which was twenty two cubits long, at the end of the holy place, which he wainscotted as the other:

both the floor and the walls with boards of cedar ;
or from the floor, including that, to the walls on each side, from wall to wall, and taking in them, they were all lined with cedar wood:

he even built [them] for it within, [even] for the oracle, [even] for
the most holy [place] ;
which explains what building is spoken of, and that the inside of that was covered with cedar from bottom to top.