All the ones I've seen (and installed) mount with brackets to the body
mounts.  The body mount bolt and lower isolator are removed, an 'L' bracket
is put between the body and the isolator, with a slightly longer bolt being
used to re-secure it.  The bar is then bolted to the vertical portion of the
'L' bracket.

Some systems then use a small bracket to triangulate the large mounting
bracket to the bottom of the rocker panel (prevents the bracket from
pivoting when it bears weight), others wrap a strap around the bar a few
inches from the mount point, and 'hang' it from the body - again, to prevent
the bar from rotating down under weight.

Are you talking about nerf bars or just side steps.  My nerf bars mount
directly to the frame which is the only place that they should be mounted.