There may be more to it but at a minimum understanding usually requires you to remember a how or why, so understanding is rooted in memory as well. Perhaps also the memory of a broader cognitive context of the world and how a given objects fit in that context. Without memory there would be no capacity for understanding.
Have you ever gained a solid understanding of a complex topic after a lot of study? When you start to forget the relevant details of the context you’ve created, your understanding fades as well. “Understanding“ is just a complex function of memory to a large degree.