They already been doing that for couple of years. Maps already knows which floor I'm on in certain malls if I have the gps+wifi on.

This does rely on an ongoing wifi mapping effort but is the most effective for regular simple smartphones without fancy sensors.

lots of phones have barometric sensors that can be used for more accurate elevation differences. sadly it seems to be a "premium" feature and i can't find it in any phone <300 dollars in price https://ieeexplore.ieee.org/document/6817923

Those sensors you are referring to are not reliable enough to tell elevation accurately. In addition to that, buildings may have different pressure to the outside

They are sufficient if you have other phones in the same vicinity at known locations.

If bobs phone is 10 meters higher than alex's phone, and they're in a 2 floor building, bob must be on the top floor and alex on the bottom.

This principle can be used at scale by people like apple and google.

i have successfully told the difference between the ground and 3rd floor with a galaxy note 3's barometric sensor(for triggering home routines only when actually at home vs just arriving at the location)

maybe that was just a particularly good sensor...

My 2013 Moto X has a barometric sensor. When graphing the raw sensor output, I can see a ~6ft elevation change with little difficulty.

GPS elevation data is pretty poor in my experience. Especially inside buildings.

To reliably triangulate elevation from GPS data you need quite a lot of satellites. Air pressure gives a better signal, but obviously needs to be calibrated for weather etc.

To some degree, but not quickly and reliably.