A good team makes improvement something its members can share without fearing replacement.

People teach each other freely only when knowing something is not the thing keeping them employed. Where being replaceable feels dangerous, everyone quietly holds back the part that makes them necessary, and the group stays as capable as its most guarded member. Security is what turns individual skill into something a whole team can use.

Make it obvious that sharing what you know is rewarded rather than spent. Ask someone to teach the rest of you their part, and thank them for it where others can hear. If you lead, say plainly that nobody becomes expendable by explaining their work.