I heard that QX gives you a mini ATP written test as part of their interview process. Sheppard air has an interview test prep variant of their software, but I am not sure if this is designed specifically for QX. How else can I best prepare for an interview at Horizon?

aviationinterviews.com has everything you need for the interview.

Don’t sweat it too hard. Smile, bring a basic knowledge of IFR and flight planning stuff and you should be fine.

Know your Jeppesen charts and the symbols on them.

Second this. I just left QX for a fractional. Aviationinterviews.com helped me with both processes. Good luck.

This would be my recommendation:

https://www.youtube.com/watch?v=WRu_-9MBpd4

so what is in the initial telephone interview, have one coming up in a few days

How did it go?

Sounds like you probably had it, but for anyone else wondering, I just had mine and it was basically them asking me what was important to me, do I want to fly a jet? where do I want to live? etc. and then they talked about each airplane, most junior bases, pathway program, benefits, the real in-person interview, training.