They have an SDK built on top of Qt Creator and it runs on OS X, Windows and Linux [1].

Linux may offer some advantages, such as access to the phone file system. OSX has a bit cumbersome ways to access it [2].

If you want to write programs for it, you can use Qt C++, QML or Python. It also has a terminal with the usual bash commands. And, of course, you can always write Android apps with tools given by Google and run those apps on Jolla.

As a niche hardware feature, the original Jolla phone has a hackable i2c bus under its back cover. Some people have tried to build keyboards, e-ink screens etc. connected to it. The specs for the bus are public [3].

I'm not sure if the new Jolla C is really meant for casual users, because you need to participate in their community device program to get one.

EDIT: removed some speculations about the community program

EDIT2: looks like the original Jolla phone isn't sold anymore

[1] https://sailfishos.org/wiki/Application_SDK_Installation

[2] https://jolla.zendesk.com/hc/en-us/articles/201440867-How-do...

[3] http://jolla.com/the-other-half-developer-kit/

The phone itself runs Linux. You can ssh into it if you want. I can't think onf any reason why you would need to use Linux for anything, but if you did then I guess you could run it in a VM.