I’m considering a stealth reverb for my soul. How does the hydraulic line get to my handlebars from inside the frame tubing?
Would I have to drill a hole in my frame?
Gear and brake lines are routed externally on the soul and there is no other holes apart from where the BB screws in.

You need an internal routed frame. Unless you want to drill it, I wouldnt!

the frame has to be designed to take a stealth post, otherwise you have to use a normal reverb as drilling holes in your bike is definitely not advised

Meh, plenty of people have done it, one brand provides a guide to DIY it. Don’t necessarily advocate it though!