Really any google search for “3d mathematics” will get you tons of examples. Most examples for one game engine will translate to Xenko very well (especially Unity). A lot of the math is hidden from you and you don’t need to worry about it too often unless you want to implement low level features. That said it doesn’t hurt to understand what is going on, so some specific subjects to look for that are related to how Xenko does some of the magic would be:
Vectors and Matrices. In particular, translation, scale and rotation matrices and how they form the view from the camera based on the positions of the objects. I have this URL bookmarked from when I was teaching myself WebGL, but the concepts are universal: http://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/ and another more complex url I have bookmarked: http://www.opengl-tutorial.org/assets/faq_quaternions/index.html
MonoGame / XNA: Xenko’s low level pieces are extremely similar to XNA (retired and replaced by MonoGame) but a lot of articles on those subjects could help for Xenko.
If you want a pretty exhaustive explanation of how game engines work, with the math involved from the most low level up (writing the math yourself, pushing data to the video card) up to modern full featured engines I highly recommend this video series. It’s for the WebGL engine BabylonJS but most of the concepts will translate to Xenko: https://mva.microsoft.com/en-US/training-courses/Introduction-to-WebGL-3D-with-HTML5-and-Babylon-js-8421?l=PjfDpUKz_4304984382
That video series got me very far and I even made some simple games using their engine. I am not trying to get you to use that engine instead, Xenko is a lot more powerful and has a lot more features (and an awesome editor) but it can be helpful with the basics.
Hope these help!