in terms of routing its something like: a route is chosen and then the actual payment is sent onion routed (like tor) where each layer/hop only sees where its come from and the next hop, so only sees the nodeid of next hop, not whole path.

payment doesnt just get shot at random from source to destination. routes are determined based on fees, no. of hops etc

someone with more experience and better skills will come along and explain it better though