In component com_users I made some small changes to layout files in /tmpl, .xml form definition files in /forms, and registration.php in com_users/controllers.
From all the reading I've done, I understand that it's not possible to override the controller, model and form xml files, at least without even more complexity that just putting changed files in mytemplate/html/... So I cannot protect my changes from Joomla updates, except by backup/restore.
What changes do I need to do to make an alternate, parallel com_users? A general description and pointer to files to change is fine.
Then where do I change Joomla to use the new component? I see that com_users is locked on the back end, presumably to prevent a user from disabling it and making it impossible for administrator to login.
Specifically, I need to understand the hooks or interfaces to/in com_user which I will need to change to/in my com_user_alt.
I am thinking I need to start by making a modified version of administrator/components/com_users/users.xml in a new installation package: mostly copies of com_user files, plus a few changes, to install?
Or is com_users is a special component that is too integrated to core to replace easily?
Sorry if this is obvious. I've read a lot of Joomla documents this past week, and there is a lot of confusing information, much for older versions.
Some day I will work through the Joomla Hello World tutorial for creating my own component and I'll full understand this :-) But I'm in a time crunch right now.
I am using Joomla 3.9.12, current as of this writing.
Thank you for your ideas.