Right inside the app: open the map picker (Maps button), then tap the info (i) icon next to any map option (National+, Hiking+, etc.) to see that map’s legend. All legends are also listed at https://support.hiiker.app/article/hiiker-map-legends/.

A couple of symbol tips: on Harvey maps, red dot lines are bridleways (shown as green dashes on OS mapping, which also mark national trails), and mysterious red circles may be the Cell Towers overlay — turn it off under Maps > Overlays.