VLC works. SMPlayer and other players recommended by Zorin documentation do not. Why not? I have tried various libraries (and am not a programmer). Recommendations please. Using Zorin 18 Core.
You properly need to install restricted codecs files (which aren't allowed to be shipped).
sudo apt install ubuntu-restricted-extras libdvdcss2
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VLC carries it's own licensing.
In terminal, please install:
sudo apt install zorin-os-restricted-extras
EDIT: Storm and I apparently posted at the same time so a quick clarification:
zorin-os-restricted-extras and ubuntu-restricted-extras are essentially the same package. 
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Thank you to both, but already have it installed.
In which package format do you have installed the other players? It may be a permissions problem if it are flatpaks. If it are flatpaks also install flatseal and set the permissions for the programs to grant access.
I have now installed Flatseal. I clicked on SMPlayer. Which options do I allow on the right (sorry, still learning)?
You can play a bit with the settings. I'm not sure what you need. Try to enable users files and/or system files at category "filesystem".
I allowed everything that was not greyed out, but still not access the DVD.
In the software store some flatpak apps also provide various additional plugins/ad-ons for that app, check if anything available for smplayer, installing those might help playing DVD.
Thank you, but nothing worked.
You could try this:
Already installed.