After the introduction of when he wrote, Mormon indicates that he wants to address what he wrote. In this case, he tells a story. When he was making his abridgment, he came to the story of king Benjamin and reached the end of a set of plates. Therefore, he had to find the next set. He searched the archive for the next installment of plates and found the set that we know as the small plates of Nephi. There is important information about Mormon’s sources contained in this terse description, which we can tease out of that description.
The first is an inference about the state of the various records in the archive. Mormon had hidden the Nephite archive in the hill Shim to preserve them. In the Nephite year AD 375, Mormon removed the records from the hill Shim as they were about to be driven from the land. This suggests that there was some haste in their removal.
The haste of the removal explains why the archive was not well-ordered as Mormon was writing. Because they were hastily retrieved and likely brought to their new location with some haste, they were disorganized. Hence Mormon had to search the archive to find what he wanted because it was not in an easily known location.
The next important aspect of the archive is that the large plates of Nephi is a name that was given to multiple sets of plates. The best guess would be that an archivist made a large number of plates, and that more were made after those were filled, much as we have seen with the small plates. In the case of the story of king Benjamin, some of the record was physically on one set of plates, and the subsequent information was on a separate physical record. That is the reason that Mormon had to go looking for the rest of the information. While logic might tell us that there could not have been a single bound set of plates that was the large plates of Nephi, the multiple bound sets of plates would appear to be confirmed with this information.