The hardware is identical! It's purely because Samsung Canada has an arrangement with the Canadian Carriers that prevents the use of eSIM.
Since you can NOT physically insert 2 sim cards, the hardware has to be different.
There's NO Hardware difference between the US & Canadian S21.
From GSM Arena.
"On Tuesday, Samsung launched its beta program for Android 12 and One UI 4. Along with the list of many new features and updates for the Samsung Galaxy S21, S21+, and S21 Ultra is support for eSIM in the US. eSIM allows a smartphone to be registered to a network without needing a SIM card. You'd only need to provide a unique eSIM number to your carrier to activate service."
That & people who have the Canadian S21 Ultra, have been able to Flash the US Betas. Not sure if the US variant did have a dual SIM if that would be possible, but thinking perhaps not, depending on what the firmware would be looking for in the way of hardware, even with Patched Odin.
There is on the European S21 series & given that the physical dimensions are the same afaik, the difference in the SIM tray itself is that SIM 1 goes on the front / top of the tray & SIM 2 goes on the back / bottom of the tray.