The offense and defense are split into groups with perhaps the linemen kept together if necessary. Defensive drills might include ripping, swimming, bull rushing, pass rushing, reading blocks, tackling, fumble recovery, and tip drills. Offensive linemen work on their various block types while the backs work to improve their specific skills. . A good scrimmage should last no longer than one hour and there are many possible recommended guidelines. One I recommend is the no contact below the waist rule.
Some coaches adhere to the rule of zero contact against the quarterback.
I think this is unfortunate because I pose that it breeds a soft quarterback who is more easily injured or rattled if suddenly exposed to the rigors of contact. Instead, I simply add a few provisions. No contact to the back other than an open handed slap. No other hand contact, no pulling or grabbing. No contact with any player's helmet or face mask.