No, usb tethering is just sending encalsulated ethernet frames over USB, the phone then delivers those with it’s normal hardware.
There is some pc hardware that can support SIM cards (I have one laptop like that I think), but I don’t know about drivers in that area
Again one more thing that works on haiku, look good
I don’t know what USB mobile modems currently do. I had one in my old laptop but it was essentially an USB serial adapter (USB-ACM) and then a modem on a (high speed) RS232 serial link. Using it would require using AT commands and probably the PPP stack which was never completed.
Maybe on newer hardware they use an USB-Ethernet profile instead? I don’t know. It could be for exemple USB-ECM (the standard profile from USB org) or RNDIS (the one defined by Microsoft and used by Android smartphones). Or even the Linux USB Ethernet Gadget one (the one defined by Linux).
PPP was never implementes or portes from freebsd Linux or something?
No, we have our own ppp stack but it was never completed