Users with custom firmware like Atmosphere can use the AIO Switch Updater to download official firmware updates directly to their SD card without connecting to Nintendo's servers.
Once you have the firmware files on your SD card, the Daybreak homebrew app is the standard tool used to safely install the update. Risks of Third-Party Downloads Download switch bios v16 bin
This is the operating system software. For emulators like Ryujinx or the now-discontinued Yuzu, you need the firmware files to run system applets and games. Users with custom firmware like Atmosphere can use