Hollow Metal doors and frames get specified in many commercial and industrial buildings (probably because the steel holds up better than wood). The website of Golden House illustrates the many options available for hollow metal products. This excellent website will give you more information about hollow metal doors and frames than you should ever need.
In recent years, we’ve been using hollow metal frames with pre-finished wood doors in many buildings. The hollow metal frames provide a durable way to complete the gypsum board openings and the pre-finished wood doors are attractive and economical. The GH doors seem to be common. Wood frames, using wood trim boards around a wood jamb and header, are also popular. Wood frames tend to be site built, except for pre-hung wood doors.