OsmAnd has a separate routing algorithm to my knowledge (was one of the first). It is definitely not GraphHopper as I created it later and prefer the routes it creates ;) but you can study our documentation to get a rough understanding how it all works: https://github.com/graphhopper/graphhopper/blob/master/docs/... and reach out in our forum for more details.

The algorithm from maps.me / Organicmaps is ported from OSRM to Android and iOS. The CTO of maps.me that did this port left maps.me in 2017 but now seems to work on Organicmaps again: https://github.com/biodranik