silent mode is just for the watch, it prevents it from making noise.
It will also silence the dings that happen with alerts on the watch, it will still vibrate for them though.
DND turns off everything. no noise, no vibrate. Alarms will still happen.
sleep mode in iOS14 just keeps the screen from coming on bright, so your watch doesn't keep lighting up as you move in bed.
for me silent mode is on all the time, and DND is on the automatic timer so it turns on overnight.
haven't done the beta, so no sleep mode