WordPress REST API
WordPress has integrated a REST API in its core. As a result, you can create a headless application or a purely HTML-driven flat CMS that talks to WordPress for dynamically fetching data or inserting real-time information.
That said, WordPress could be installed separately and you could use the platform as a separate solution connected to different software applications.
Alternatively, there are additional callbacks that let you connect to other services – including smartwatches or other smart devices. Just familiarize yourself with the general code architecture of WordPress.
3rd Party Services
WordPress supports thousands of different 3rd party services. Usually, most external solutions create a bridge plugin that connects a SaaS or an external solution to WordPress. Another alternative is including a JS snippet communicating with the platform (a popular example being Google Analytics) or even an iframe in certain cases.
You can implement additional features that could be queried via HTTP requests.
If you run WordPress on a VPS, a dedicated server, or in the cloud (having root access), you can build various scripts that run directly on your “physical” server.
As a result, running helper tools built in Ruby, Python, Java – or anything else – can be connected to a WordPress app as well.
We use a bunch of Ruby and Python tools for deployments, resizing images on-the-fly, processing data to various ERPs, providing various forms of authentication, etc.
WordPress on C# And .NET
While I wouldn’t risk going there, WordPress could technically run on a .NET stack.
I have some hardcore .NET friends who have managed to pull this off and it seems to work (for the most part). There are certain MVC Frameworks for traditional WordPress projects, too.
As a rule of thumb, sticking to PHP and JS would be your best bet. Large applications on top of WordPress regularly utilize 3rd party solutions and tools built in other languages as well.