modern GNSS chipsets might as well be magic. i just got a lock with 16s TTFF in a bathroom with no windows in a house with a metre thick reinforced concrete walls
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(I don't know much about modern chipsets...)
Is this just new schemes / good antenna / signal processing, or is this in partnership with A-GPS type shenanigans?
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modern smartphone-grade antennas are awful but the signal processing is quite good. agps also helps a great deal for ttff
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Indeed - A-GPS gives you (relatively speaking) instant almanac, and an approximate location via the cell network... which definately facilitates magic when compared to previous technologies!
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Snapdragon deals with SUPL inside the cellular baseband and GPS since it's based on the cellular network. OS is responsible for choosing the mode of operation and the servers. The fallbacks for carriers not overriding the defaults are set for AOSP here:
android.googlesource.com/platform/frame
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You should try using GPS in airplane mode to see how well it works without SUPL. In my experience, it takes so much longer that it's hard to tell it's even going to work at all. I think it will still use the supplementary GPS data downloaded by the OS though, if that's available.
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So you could also try without Wi-Fi enabled either and without already having a copy of the supplementary data and see how well it actually works based on raw GNSS.
It should take a substantially longer time in airplane mode even with the supplementary data available though.