It sounds as if you are referring to orchard mason bees, Latin name Osmia lignaria. For instructions on how to build a block nest for them, see the following publication title "How to Raise and Manage Orchard Mason Bees for the Home Garden," authored by North Carolina Extension:
HOW TO RAISE AND MANAGE ORCHARD MASON BEES
Orchard mason bees need a source of moist soil or mud to construct their nests, so you may need to provide that if there isn't a source nearby. A tub of muddy soil should suffice. If you prefer not to build your own nesting block, they are available for purchase.
