How do I keep a crossbody bag secure in crowded cities?

Across r/travel and r/solofemaletravellers, the most-repeated advice is to wear the bag in front in dense crowds, use bags with the main zipper facing your body, and prefer designs with locking or hidden zippers (Travelon Classic Messenger, Travelon Heritage). RFID blocking is a nice-to-have but secondary to physical access control.