That used to be the case but it has changed now. Probably depends on the phone broadband chipset used.

These days you can passthru your WiFi or even a wired connection (via USB to a connected PC or a Ethernet-to-USB adapter) via a Hotspot.

You can on Android, and have for some time IIRC. This is how I get free wifi on my computer by passing it through my T-Mobile phone.

I absolutely can, that's how I connect my ps4 to my uni accommodation internet as it's mschapv2 or smth the ps4 can't connect to. I know it's definitely using the wifi and not my mobile data as my data usage for the day is unchanged after I've downloaded a game.

Cheap Chinese android phone from 2020 (or maybe 2021 can't remember).

Correct. My GL-E750 Mudi has a repeater function:

https://docs.gl-inet.com/router/en/3/setup/gl-e750/internet/...

My iPhone does not.

Depends on the phone OS and wifi chipset Some Pixel phones can do it last I checked, which was about a year ago. It's a fragile dependency. For example, a Samsung with the same wifi chip didn't work.