How do I convert my mac mini into a remote server?
For private use, a product like TeamViewer could be used, but for a more integrated solution you would need more knowledge (which can be attained for free) to setup and maintain a server.

There used to be a simple solution using iCloud’s Back to my Mac, but that is phased out as far as I know. (https://support.apple.com/en-gb/HT208922)

Running a remote desktop session without strong encryption is not recommended and leaves you vulnerable to assorted attacks, so a self-supported setup will be more involved than you might imagine.