In Book 1, Ch 28, you implemented BFS and DFS on adjacency lists. BFS uses a queue and explores level by level. DFS uses a stack (or recursive call stack) and explores one path fully before backtracking.
This chapter traces T3 (Graphs) using T4 (Queues): the frontier queue of BFS and the explicit or implicit stack of DFS are the data structures that define the traversal order. In Book 3, the graph thread continues with network topology, service mesh routing, and social graph storage at scale.