Also, you probably don’t have the right to make your own mobile modem unless you’re fine with it needing its own car battery and alternator thanks to Qualcomm’s patents.

Standards patents must be made available to license. Apple tried really hard to invest in a competing cellular modem provider (Intel) and gave up. Turns out, they’re really hard to build, and that’s why you can’t build one. Not patents.

Apple did purchase Intel's modem division[0], so while they temporarily have started purchasing from Qualcomm, maybe they're throwing more R&D at it than Intel did to actually compete with Qualcomm within the next few years.

0: https://www.apple.com/newsroom/2019/07/apple-to-acquire-the-...

I'm not following you. What's to stop you setting up your own local network, entirely under your control?

For a LAN: Nothing, but you won't be able to communicate to anyone outside of it. You also won't be able to access it away from wherever you set it up.

For the primary networks at issue here, cellular networks: The FCC, and regulations restricting what devices can access which EM frequencies legally.

Nit: it is possible to build a private LTE/5G network if you use appropriate spectrum reserved for that purpose:

* https://www.sierrawireless.com/iot-blog/what-are-private-lte...

Not sure about connectivity to the larger / global SS7 network.

So, you don't own other people's networks? Right.