How to Downgrade and Lock Packages With dnf in RHEL Systems

You ran dnf update, and now something has stopped working. Instead of spending hours troubleshooting, you just want to go back to the package version that was working before.
This happens more often than most Linux administrators would like to admit. Maybe a new Nginx release introduced a default configuration change that broke your virtual hosts. A Python library update changed an API that your internal scripts rely on. Or perhaps a kernel update no longer works with a third-party driver.