The book of Mosiah is the most complex writing task that Mormon undertook. There are multiple threads of stories that are happening at similar times, but at different places. Telling one story necessarily put Mormon’s story out of sync with the others. Thus, there are flashbacks within flashbacks.
Mormon began with the story of Ammon finding the people of Limhi. That event led to the explanation of who that people were, hence the story of Zeniff, then Noah. Telling Noah’s story required that we meet Abinadi, and that triggered the story of Alma. After introducing Alma, Mormon set his story aside to finish with Noah and then move to Limhi. When that story caught up to the story of Ammon, he returned to the story of Alma, which began before Limhi became king. It also occurred in a different location, and was recorded on a different record.
Even the telling of the complexity is complex. The task Mormon had was to pull all of those stories together. They would not be truly unified until both Limhi’s people and Alma’s people arrived in Zarahemla.
At this point, Mormon is shifting to Alma’s story, so we move back in time. Mormon had told us that Noah attempted to find Alma’s people, but failed (see Mosiah 18:34 and 19:1). That ending was given from Noah’s viewpoint. Now he picks up right after he left off, but from Alma’s point of view.