Curious about the RoutePlanning feature

Hello, I'm working on an application that needs to provide runners with routes of specified length. The routeing for cycle.travel is really good, and I would love to see what steps were taken to accomplish it. 

What open source software was used, how was it augmented?

How large is the uncompressed network that was generated from the OSM data, and how was that built?

How did you approach creating the routes?

I've done some querying but I can't seem to find any blog posts or other information about solving this problem, it would also be very helpful to maybe get be pointed to some resources.  

Thank you.