Pretty sure Zigbee also defines a control layers, so there's such a thing as a standard switch or power meter, or thermostat which can be controlled by any generic piece of software.
A lot of devices are not compliant though and either have extra functionality exposed in a nonstandard way, or don't comform to the standard well enough.
So your Zigbee light switch will probably workout any fuss regardless of vendor but more complex devices might not.
That's exactly the problem, there was no standard protocol for communication over Zigbee. Manufacturers could implement whatever they wanted on top of it and put the Zigbee logo, like you can put the WiFi logo on a device that speaks a proprietary protocol over WiFi. You bought into an ecosystem and if you wanted a device from outside of it, you needed another hub.
> So your Zigbee light switch will probably workout any fuss
Big "depends". Out of the box it will only work for the few manufacturers who look to be compatible with some other hubs. I tested a lot of basic devices (simple switches or bulbs) with various hubs with little success. Philips, IKEA, Bosch, Tuya, Aqara, Osram, etc. Couldn't discover, add, or control them properly without the corresponding hub.
If you use a device with HA and a Zigbee stick (router/coordinator) then you benefit from a lot of development done in the background to "translate" between all the variations. But that's not something non-techies want to deal with, it's too much of a hassle.
This is the problem that Matter solves. Certified devices must implement the standard communication protocol over the network of choice. So no matter the manufacturer, if I see a Matter logo I know the device will work with my Matter.
I guess I got into smart homes quite late, and have the benefit of accumulated knowledge and got to skip out on decades of half-working devices.
Nowadays, from what I've seen, Zigbee devices seem to be based on a couple of good ICs (and software stacks), which are inexpensive and battle tested.
Forgive me, I don't have a comprehensive experience on what standard does exactly what and how, but from what I've read Matter is a no go for homemade stuff - you need to go through cert for the hubs to talk to you.
As for out of the box support, I've read mixed things - I've heard that for many devices that are Matter compatible (more particularly, the Aqara W100 thermostat), you can't do everything the device supports via Matter that you can do with more proprietary APIs. Considering many of these companies are pushing their own ecosystems, they have an incentive to keep an advantage. Many people are already going for using Thread without Matter.
I feel like Matter is like the joke where they try to solve N competing standards by adding another one, and experience shows this isnt the way to go. The way to go is what Home Assistant does - they keep an open ecosystem with support for plugins, and for example for Zigbee, they implement the 'quirks' of your device into the framework, so you don't have to deal with it.
Having bought a few Matter devices now, I have discovered that, in practise, Matter is just as full of vendor extensions as ZigBee, and the quirks ecosystem that allows for interoperability despite vendor extensions is far less mature than with ZigBee.
Maybe this will get better with time, but we're half a decade into the Matter era and the end-user experience is _worse_ than with ZigBee. In that sense, Matter has failed.
A lot of devices are not compliant though and either have extra functionality exposed in a nonstandard way, or don't comform to the standard well enough.
So your Zigbee light switch will probably workout any fuss regardless of vendor but more complex devices might not.