Where in the lectures does the first image come from? The second set of images showing how “non-max suppression” works is specific to how the YOLO algorithm works, which is the main object detection algorithm that Prof Ng describes, once he’s given the overview including earlier techniques like “sliding windows”.
There are lots of very detailed threads on the forum that explore how YOLO works and how it is trained. Here’s a good one to get started down that path. And here’s one that talks about non-max suppression and the role of anchor boxes (not the same as bounding boxes).