The main way is that literally zero of these watches actually meet the standards that the cell networks require of a cell phone. Every single one of them has a carrier exemption or a lower standard to adhere to, because it turns out that putting a cell phone's RF package into a watch is super hard, both because of size and the various negative effects of the human body on radio signals. This affects cell phones too of course, but less so (remember the iPhone 4 and how we were "holding it wrong"?).

Another way is that watch chipsets are distinct from cell phone chipsets in that they make a variety of compromises unique to wearable requirements. Apple may be an exception here, you can't get a spec sheet for their chip, but for the other providers their wearable chipsets are generations behind anything they sell for a cell phone and are compromised in terms of power. Interestingly even watches (Apple, Samsung soon) that support 5G are running a dumbed down version of 5G that was created specifically to support the wearables and IoT market.

It gets even stranger in software. A text showing up on your watch might have arrived two completely different ways depending on whether it's an iMessage or a regular text and you can't tell which. The watch often doesn't even have its own number -- it's borrowing your phone's. IOW, it's not a tiny phone doing phone things, it's a companion device trying to fake it.