If you want an easy way to use SX OS and have trouble understanding how to select the payload, then SX Tools is for you. No need to download the SX Loader payload and manually select it from the app.
Install the apk to an OTG capable android phone.
Connect the Switch to the phone.
On the first use, the app will ask for permission to "use USB device" when the Switch is connected. Check the "Use by default…" and press "OK"
SX OS will boot now.
That's it. You don't even need to open the app to use it. Just install it and connect the Switch. Even when the phone is locked, it will work.