The nRF5x Series chips are flash-based SoCs, and as such they represent the most flexible solution available. A key feature of the nRF5x Series and their associated software architecture and S-Series SoftDevices is the possibility for Over-The-Air Device Firmware Upgrade (OTA-DFU) over Bluetooth LE.
The DFU application allows to update nRF5 devices using Secure or Legacy DFU bootloader from nRF5 SDK version 4.3 or newer.
The firmware should be in .zip file format, prepared using "nrf util" (https://github.com/NordicSemiconductor/pc-nrfutil).