Yes, and there is even a great FLOSS hub software Home Assistant [0] (though to be fair, it requires some tinkering compared to the proprietary systems) and if you run it on a PI or small server, you might as well get a conbee II which is a USB stick that adds Zigbee support directly :)

[0]: https://www.home-assistant.io/

Yep, you can do one without the other.

The bigger hassle seems to be different zigbee hubs that have varying levels of compatibility, so while you can pair the devices you can't get the data you want.

I've just been down the research rabbit hole, and about to try using zigbee2mqtt to bypass this nonsense (this also then means the only internet connectivity will be through a device you control)

Also systems like Philips Hue use ZigBee protocol. We have Hue lights at home and I'm planning to plug a ConBee II to my router for the Home Assistant to see how well I can run the lights with another hub.

Philips Hue hub talks to diagnostics.meethue.com all the time, and it gets kind of annoying already in the pihole stats.

there's also a difference between IoT and "look ma I wired an ESP32 to a DHT22 and sent data to timstream on my NAS". The latter is entirely you. WiFI isn't the issue, it's running cloud based hardware that can't work without someone's servers.

It’s a wireless standard, like Bluetooth Low Energy or Wi-Fi, not a home automation standard. Actually predates BLE.