Here is the answer of your each questions :
Q: How can I tell what version of SharePoint my "personal site" is running? I suspect "SharePoint 2016", On-premises. How can I tell?
Answer: To get SharePoint on-premise version type the below URL in the browser.
To check the version of SharePoint Online, use below ...
You can create a workflow or Microsoft flow/power automate to update the Email address field based on the Person or Group field.
If you are not going to use those email address values later (you don't need to save emails in column) and you just want to show the email in SharePoint list view then you can also achieve it using JSON ...