Macbook Pro 16" fried all USB-C ports after using a Samsung 43" monitor
Without putting hands on with the proper test and diagnostic equipment, there is absolutely no way to tell.

I used the original USB-C cable from Samsung, not anything cheap, so why did this happen?

Until you test the cable, you have no idea whether or not it's at fault.  However, if you have any doubt as to the functionality and reliability of said cable, cut it in half (to prevent it from being used by others) and throw it away.  Replace it with another from a known good, reputable brand.

This could have been a failure on your monitor's part - it just dumped way to much power into your Mac.

This could have been a failure on your Mac's part.

USB controller that negotiates power/charging died in some way
The SMC (and related circuitry) could have had a component failure




I'm mainly asking because I got the laptop fixed but I'm afraid of hooking it back up to the monitor.

If I had to guess, my money would be on the Mac failing and not your monitor.  However, you can get a rather inexpensive USB-C power meter to &quot;see&quot; what your laptop's drawing and what your monitor (or other power source) is delivering.  These run from $20 to $40USD online.

It's it &quot;pro grade?&quot;  No.  Those cost hundreds and would be way overkill for what you want.  But it's &quot;good enough.&quot;  We're not looking to measure microvolts or check for negotiation packets.  It will tell you, however, if something is majorly wrong so you can take preventative or corrective action.