The basic idea is that you are out and about (so your home WiFi is not an option) and your phone has a mobile data connection but your other device (in this case the Librem 11) has no mobile capability at all … so you want to use tethering in order to get internet access on the other device.
There are three potential communications technologies to use for tethering
- USB
- WiFi
- Bluetooth
WiFi is in some respects the most convenient (doesn’t require a cable) and should be adequately secure.
So the Librem 5 puts its WiFi card in Access Point mode (aka Hotspot mode) and the Librem 5 routes traffic from the WiFi interface to the mobile data interface - and all the Librem 11 has to do is associate with the WAP that the Librem 5 advertises. (I believe this option is on the GUI of the Librem 5.)
Bluetooth not further discussed here. I have no idea whether it works for tethering when using the Librem 5 and what would be involved. It’s comparatively slow anyway.