A waterproof bag works by closing every path water could take into it. There are three paths, and each needs its own solution.
Through the fabric. Waterproof bags use fabrics with a continuous plastic layer, PVC tarpaulin or TPU-laminated fabric, which water cannot pass through, however long it is exposed.
Through the seams. Sewing punches holes in the fabric, so waterproof bags join panels by high-frequency welding instead. The machine heats the PVC or TPU from within and presses the layers together until they fuse into one piece, leaving no holes for water to follow. Straps and handles are sewn onto welded reinforcing patches outside the waterproof layer.
Through the opening. A roll-top works by folding the opening over itself at least three times, so water would have to travel through several tight folds before reaching the inside; the buckles hold the roll in place. An airtight zip uses interlocking teeth with a rubberised seal that closes against an end stop. Both can be rated for immersion when closed correctly.
A sealed bag also traps air, which is why many dry bags float if they fall in the water. To learn more about the welding process, see our guide to RF welding and our comparison of welded and sewn bags.