In short it’s a way to share network access to storage across MacOS/Linux/Windows.
MacOS switched from AFS to SMB (as the default file sharing / network storage protocol) a few years ago as it was clear that was how everything was headed - though iOS and MacOS also have native support for NFS.
On linux, you can use samba to create SMB shares that will be available to your iOS device.
It’s a lot of configuration though - so maybe not the best choice.
As for Nextcloud - indeed you can use it in your local network without making it available on your WAN connection. That’s how we use it here.
When we need it remotely - we VPN into our home network. But no exposed ports. :)