You’ve got the idea.
A SmartThings hub is essentially one plastic box with multiple IOT radios ( for example, some have a Wi-Fi radio, some don’t; the dual logo ADT/SmartThings model has an additional radio which only works with the dual logo sensors, the European models have radios on the European regional frequency, etc).
But as of this writing, all ST hubs have both a zigbee radio and a zwave radio. They are certified to establish networks of that protocol by the independent third-party organizations.
So that means you can add any certified zwave end device to the zwave network established by your smartthings hub (this process is called “including“ in zwave) and you will get at least “basic“ (that’s a specific zwave term in this context) functionality.
Zigbee is a little more complicated because the profiles also have to match. See the FAQ:
FAQ: Zigbee Application Profiles, or why not all zigbee devices work with SmartThings
(In zigbee, the process is called “joining.“ But SmartThings just says “add a device“ regardless of the protocol.)
Devices of any other protocols are not added directly to the hub, but get added at the account level.
Since it sounds like you have a strong technical background, you might find the welcome FAQ in the community – created wiki also helpful. 