The process is the same for all the armies. It was done at the time of the creation of the Paths of Magic and is open to minor adjustments if needed during the LAB creation process.
Based on the in-world "magicness" of the factions the RT determined how many Paths a book should have access to. At the same time, the BGT orders all the paths of Magic from most appropriate to least appropriate for each faction and for each spellcaster in the faction should that differ from the overall appropriateness. To determine whether a Path is more or less appropriate underlying philosophies of the Path are compared to the underlying philosophies of the FAction/Spellcaster.
Once those two tasks done BGT's appropriateness list is cut off with the number of paths allowed by the RT and that is how you get the path access.
There is some more fiddling to ensure that there are as few path allocation patterns as possible, as well as ensuring certain paths are not overrepresented.