Does that actually work? I don't think you can both have hotspot on and be connected to another network.
Yes, it has actually worked starting with the Pixel 3.
It's called Dual-Band Simultaneous or "STA+AP" (Station + Access Point) concurrency that can bridge an existing wifi connection to an access point to other devices via a hotspot.
Yes it works. Now you can also tether via USB. Both of them have worked flawlessly for me recently.
It seems to be only on certain devices feature(?): on my Pixel it worked, Samsung phone just says "sorry, can't do that".
Works fine, yup.
Most newer (or at least new + expensive) phones can share their wifi connection via hotspot. 2.4gh only though I think.