Does rebar or forms get placed first?

The rebar is placed first. Sometimes a side of the form will go up, than the rebar. This is primarily because the bar being placed above must be tied to the bar below. With forms up, the bar could not get tied into place. The only bar placed after forms might be caissons, drilled piles or sonotubes where there are stay in place forms retaining the surrounding soils for concrete placement.