In Memory-Map, Topo maps are typically published as large seamless maps covering an entire state or country.
While it is possible to download the entire map, it is often more convenient to download just the region you want to visit. This video shows how, for PC and Android. iOS and Mac are similar to Android
Note that the video shows an Android device, the process is identical on iOS and Mac. As is often the case, the PC app is a little different: Here you have to right-click on the route, select Digital Map Store, then select the map you want to download.