Hi, I think there's a mashup of terms here. Pegs historically were used to cover the heads of fasteners. Before that, a similar look was probably a result of using wood dowels to fasten pieces of wood together. Groove is usually a term used in conjunction with tongue, as in tongue and groove.
Pre-bored flooring isn't as prevalent as it once was but unbored flooring can be purchased and installed. After an acclimation period, one makes templates to mark the boards, usually at the ends. After marking, you use an appropriate bit to bore for the peg. If you're screwing the floor down, you drill for screws, install the screws, insert the pegs in the bored holes and sand and finish the floor as usual. It's generally less expensive to buy pegs rather than making them with a plug cutter and a drill press.
One can have wood with plugs that do not cover a fastener so they are strictly cosmetic. They can be used on engineered flooring or solid flooring. Pegs can be in the same wood or a complementary wood. Pegs can be end grain or flat grain.
Any full service wood floor company should be able to do a pegged floor.
Q