I understand how to use “Virtualmin / Server Configuration / Website Options / PHP Script Execution Mode” on an existing virtual server to change the script execution mode, but
How is the global default changed; i.e., all new servers would use the specified chosen value of “Apache mod_php” or “FCGId (run as virtual server owner)”, etc.
Is this how “server templates” come into play? If the template changes, then do all servers using that template change? Or just those create in the future, from that template?
Changing Server Templates does not affect existing domain configuration, only new ones. But, yes, Server Templates is how you decide what you want your defaults to be for newly created servers. It also allows you to have more than one set of “defaults” based on what the purpose of a given domain will be; or you can have different templates for different resellers, domain owners, or whatever.
You can do mass changes for many things using the virtualmin CLI command (which can be scripted in whatever scripting language you prefer, though it’s often possible to do most things with a one-liner in bash).