settings -> sound -> do not disturb -> automatic rules
Here you can add as many time framed rules as you like. My needs are pretty simple, I just have a couple of recurring weekly times where I want to set DND and vibrate notifications only. Here is a screen shot of one of them:
Notes: 1) You can set it to recur based on day of week for any or all of the seven weekdays 2) start and end times for DND period. 3) DND style: Priority notifications only, Alarm notifications only, or Total Silence (not sure how useful the first two are to most folks but they are there if you want to investigate... doctors on call maybe? I am guessing at a potential use case here). 4) Alarm overriding end-time maybe for those that want an alarm sent first to kill the "DND notification firewall" and then sender follows with actual voice or text call maybe? Again, not real sure what real world use of this setting is... feel free to enlighten me in the comments.
One final note of interest here: I also discovered you don't need to try and slide the volume down with the touch screen or hold the volume down button. Doing it with the volume down key means waiting for the auto-repeat to kick in and slowly reduce the volume to vibrate while the annoying advertising electronic "music" in my case fades away like a bad sound effect in a B movie. Android already has a single touch mute if you know the secret handshake...
1) power up device and unlock screen
2) volume down once only
3) touch the bell graphic on the left of the volume slider.... boom muted.
This single touch mute has been around since at least android nougat (5.1) BTW.