Can someone please confirm that my understanding of this is accurate?
In the this link (What is the Mixed Reality Toolkit | Mixed Reality Toolkit Documentation) it says the following. Does this mean when I build the app, someone with HTC vive will automatically be able to use it (without me having to use SteamVR plugin) ? Also, what about the controllers, will vive/index controllers also work?
The MRTK is just a framework, you need to add the SDK of the support you need, like SteamVR and Oculus plug-in.
About the interactions, is very simple, the MRTK handle every input and provide a bunch of common events.
You should take a look at this documentation.
If you’re trying to create an only VR application I don’t think the MRTK is the best toolkit for that.
If that help you can aslo see VRTK, XRTK and MRTK-Quest (Extension of the MRTK optimized for Oculus Rift S/Quest).