How to Provide Private Remote Access to macOS 10.15.7

What do the heirs want from the Mac Mini? Why not have the heir that has the Mini copy the Documents folder, Pictures folder, Music folder and any other files that the others might want access to to 3 bare SSDs with SATA to USB adaptors like this one:

and send one to each heir? (have them send the money first to the heir with the Mini). That way they can have direct access to the files without the hassle of sharing, etc.

This is a probably going to be a bad idea, whether for basic server management considerations, or for maintaining server security, or just dealing with near-inevitable squabbles among heirs. Is the server administrator willing and able to accept this (and make no mistake) work, including supporting access and clients for the rest of y’all?

As for your question, I’ve posted the usual remote-access setup options fairly regularly over in the macOS Server community. Search for my nick, and keywords such as: VPN, DMZ, firewall, port-forwarding. (I can dig up a few of my representative postings here too, if your designated server administrator really wants to follow this path.)

You’ll need either port forwarding in the firewall (for either screen sharing or a VPN pass-through), or mid-grade firewall with an embedded VPN server. You'll need either a fixed IP address and DNS, or a means to update a dynamic DNS (DDNS) service to allow remote clients to obtain the IP address of the firewall.

Or you’ll need to install an app that punches through the firewall for you.

In short, you’re seeking to learn how to administer a server. And a server that will get attacked.

If that seems like, well, work, what I’d suggest is either hosting the content y’all care about such as photos, and providing each heir with a copy of the Mac contents. There are various options here, including (temporarily) hosting the data elsewhere.

As for security, I would be exceeeeeeeeeeedingly cautious about setting up wide-open port-forwarding for SMB file services or the built-in screen sharing or otherwise, as every miscreant on the whole ‘net will be poking at and trying to gain access into the Mac, and you’re one compromised password away from catastrophe. Using a VPN (a classic VPN, not the widely-hyped first-few-hops commercial metadata-collection VPNs) gives you some control over access into the server. But now you’re administering a VPN.

I’d likely also partition the local network with the Mac, so that any potential breach of the Mac (or any heir that won’t stay on the Mac) doesn’t get easily extended into other local gear on the rest of the local network. This network isolation is variously called a DMZ.

Which is a lot of words to ask… What your goal for this Mac? Setting up server timesharing? Sharing photos? Archiving contents? Something else?

Great, then you can setup File Sharing...

Use file sharing on Mac - Apple Support

Then you'll need Port Forwarding on the router, like this but SMB instead of AFP.

Do you know the passed individuals admin password?

What kind of Mac & vintage is it?