Hi @Luke86

To setup high availability, you would setup multiple database servers, storage servers and application servers. Hosted in different data centers.
You cannot share a database like MySQL across multiple active database servers just on the file level. But you would run it in a master-slave or master-master type of setup.
For storage, you would run something like GlusterFS, so you could share the media storage across multiple servers.
And then you would run multiple application servers running the Apache/Nginx and PHP. And the application server could even be split up even further, by putting Apache/Nginx on one and PHP on another.

All in all, it’s a little complicated, but aiming for 100% uptime doesn’t come easy.
A simple way having a lot of uptime on a static website would be to use a cache service like CloudFlare (or setup your own), which would serve the website even if the backend (WordPress) was down, since it would serve all known pages from it’s cache.