How do you pre-drill side paths ? Isn't being able to drill the hard part ? I am not saying it's dumb but from what I know about the industry (almost nothing) it does not seem simple at all

They need to be able to steer drills for all kinds of reasons: https://www.youtube.com/watch?v=JAhdb7dKQpU

Actual answer seems to be drilling side holes also involves removing the pipe. https://news.ycombinator.com/item?id=43371776

The drill is on the end of what amounts to a several km long rope. I’m pretty sure a side hole would introduce friction with the side that wasn’t there before, making it easy to stick going in and out. I think if the obvious solution worked then people would already be doing it, cause they’d save multiple millions of dollars.

They can do side drills, but apparently that also requires removing the pipe which is the bit I was missing.

https://news.ycombinator.com/item?id=43371776

Ah yep that does explain it well.