I want to customize the Auth Page so that when logging into the admin panel, a letter with a one-time code is sent to the specified mail. How can this be implemented?
System Information
- Strapi Version: 3.4.1
I don’t think you can do it straight off without messing with the core files.
You can of course do something like Authelia and put it in front of the actual page.
how to do this by modifying the core files?
As bad as it sounds if you not sure what file you should not be doing it hence “core” files 
But I guess you have to overwrite the packages or files here / Extend them
It might be better to wait for V4 coming out which is soon 
I believe and possible people or built in has a plugin to work with what you wanted.
It’s also been discussed here 2FA (via users & permissions plugin) - Roadmap | Product Roadmap
1 Like