Paintings are either on canvas, or they are “boarded, banded, and waxed.”

Boarded, banded and waxed means the painting is mounted to 3/4" premium plywood, the edges of the plywood are covered with a vinyl banding, then the painting is covered with 2 coats of wax to seal against dust and finger prints.

Watercolors painted on birch plywood then sealed with satin urethane

Watercolor painted on a prepared canvas then sealed with a satin urethane.

Yes, the smaller ones use a picture hanger.

The larger ones use picture wire.

Yes, on a piece by piece basis.