most software now uses multiple GNSS at the same time?

That's kinda what RTK does, you have a ground station and transfer correction data for the visible satellites from it to your GNSS receiver. It doesn't transmit a GNSS signal itself but functionally the effect is vaguely comparable to a "satellite on the ground", without interfering on the actual signals.

Some places have free RTK networks their citizens can use. Michigan's department of transportation, for instance, runs a CORS network that anyone can request access to. (At least as far as I can tell. The signup form doesn't ask for affiliation or payment.) It's just distributed over the internet, the assumption being that you probably have internet access already.

One of these days I'll figure out how to set up a free NTRIP caster on my Galmon station so it can do double-duty. The trick then is advertising and discovery.

It would be lovely to have, say, a standard wifi SSID or a standard LORA channel that your local corrections network would broadcast on. That way you could have a large number of client devices not each needing their own internet access SIM card or whatever. I wonder if the corrections stream would fit into an FM RDS payload or something.

Trouble is, there's so much money in the L-band corrections services, and so little money in replacing them for free...

Oh, yeah, the cryptocurrency folks have weighed in, there's a thing called "goodnet" which appears to be microtransactions in exchange for NTRIP streams over some medium. I haven't looked further into it.

GPS beacons would be stupidly expensive, as you'd need tens of thousands of them per city, and each one of them would need a very accurate atomic clock.

It would make far more sense (but still unviable) to go for Eurobalise-style RFID tags embedded in the road surface.

Fun thing (?), I was excited on my last trip to Japan so I could test QZSS with my Pixel 9a, that supports it, and my GPS experience in urban canyons, specially in Shibuya, was terribly bad.

Even when in GPS Test or GPS Lock tools it was showing better than 3 meter horizontal accuracy, and a multitude of locked satellites, including some QZSS, the location would usually be 30 to 50 meters away. The first days I though I had lost all my capacity to navigate Tokyo, then I noticed the GPS was gas-lightning me.

I tried removing the phone case, changing GPS settings... and I had no luck.

Almost, they use accelerometers and gyros, 'dead reckoning' is the keyword to look for. The wheels are a bit unreliable because the diameter changes slightly with pressure and temperature.

Yup! Otherwise your navigation would become totally useless the second you entered a tunnel.

Dead reckoning can get quite accurate once you realise that cars drive on roads, so if you have a reasonably up-to-date map you can use turns and corners to "snap" back to the road and reset a good bunch of your accumulated error.

"with enough integration time" — the article is about live navigation, which generally can't afford that.