Here are some videos that show the underpainting technique in the early part of the videos. Yes, there are being underpainted with oil paints and it is a different technique to underpainting with acrylics. The finished underpainting has the same end result though and it is the end result that is important.
When using Oils, the burnt umber underpaint is applied in various thicknesses and can be thinned out and even rubbed away to produce lighter areas.
When using acrylics, layers of thinned acrylic paint are added to build depth.
Another video on Burnt Umber, multi-layer underpainting
And yet another underpainting demo
And another underpainting video