This is TurrisOS, not vanilla OpenWRT. Root is a read-write btrfs volume. You can install packages, you can change whatever you want. You can even run container

This is TurrisOS, not vanilla OpenWRT. Root is a read-write btrfs volume. You can install packages, you can change whatever you want. You can even run containers on it, although it's highly discouraged precisely to prevent it from wearing out too soon. In any case, on vanilla OpenWRT the overlay is rw and also gets written to the flash. It sees a lot less activity than on TurrisOS but it exists, otherwise it couldn't persist your config between reboots.