Technically there’s no such thing as a “matter hub.” That’s because matter exists at a level above the network communication level. So you have “matter over Wi-Fi“, “matter over thread,” and “matter via bridge.”
What matter does have are the following:
A) “Matter commissioner.” Almost always an app. This is what you use to add a matter device to an account. The smartthings app is a matter commissioner. So is the Apple home app. So is the Alexa app, although at the time of this writing, it didn’t have as many features as the others. There are several others, including LG’s THINQ app used with their televisions and smart appliances.
B) “Matter Controller.” This is probably what you were thinking of as a “matter hub“. This handles the message traffic between matter devices. So far most home automation platforms have been adding the Matter controller functionality to their hubs. smartthings V2, smartthings V3, the Aeotec “works as a Smartthings Hub, “ the SmartThings Station are all matter controllers. So is the apple HomePod mini and Apple TV box, some Amazon echo devices, and some Google home devices.
You have to have a matter controller to use matter devices with any platform. You can’t use the smartthings hub to act as a matter controller for the Alexa app, which may be where you got confused. Each platform has to have its own matter controller or it can’t use matter.
In smartthings, for example, the vast majority of smartthings customers at the time of this writing did not have a standalone hub. If they have a smart appliance like the refrigerator, they may be able to add a dongle which can act as a matter controller. And some of the Samsung smart TV models have a built-in hub, which can also act as a matter controller. But if you had, say, just a galaxy phone and a Samsung robot vacuum, you would be using the smartthings app, but you wouldn’t have any SmartThings matter controller, so you wouldn’t be able to use matter devices in a smartthings app. Even if you had a Matter controller from a different brand, like a HomePod mini, or a Google Home model that was a Matter Controller for Google.
C) “Matter bridge.” This is a device specifically made to allow one platform to connect devices joined to its matter controller to a different platform using that platform’s own matter controller. There are only a few of these right now.
None of the smartthings hubs act as a Matter bridge. The switchbot 2 hub and a number of the Aqara hub models are matter Bridges. You can add the bridge to your smartthings account, and it will bring along with it some of its connected devices (but not all models or all features.).
For example, I have added my Aqara Hub M2 hub to smartthings as a matter bridge and it brought in an aqara smart button with it.
Matter bridges work in a very similar way to the way the Phillips hue bridge worked before matter. You connect individual devices to the bridge. Then you add the bridge to your home automation account and it brings those devices with it. And just like the Philips hue bridge, a Matter bridge can be connected to multiple home automation platforms at the same time.
D) “Matter binding.” the independent third-party standard developing matter also intended for it to be possible to bind two Matter Devices together, like a Sensor and a smart bulb, without needing a Matter controller. However, as far as I know, nobody has actually implemented this yet. 
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So, at the time of this writing, to use a matter over thread smart bulb with Apple Home, SmartThings, Alexa, and Google Home simultaneously you would still need a separate Matter commissioner and matter controller for each of those four individual platforms. Four platforms: four matter commissioners, four matter controllers.
But each of those platforms would be able to control the same smart bulb, so for your everyday use in terms of device control and creating automations, you could use any of the four apps that you wanted (and any or all of the four voice assistants) after the device has been set up.
Matter doesn’t actually change anything at the network protocol level. Wi-Fi is still Wi-Fi, Thread is Still Thread, zigbee is still Zigbee. So it doesn’t change the number of hubs you need. That’s the underlying architecture of how messages are sent back-and-forth electronically.
What it changes is the next layer up where app control and voice assistants work. (This is called the “application layer” in network engineering.) what’s new is being able to use any or all of the participating user interfaces with the same device at the same time, again, once the device has been set up.