The sidebar (on a phone: Menu in the tab bar) has Home, Tasks, Calendar, Ask AI, Graph, Notes, Notebooks, Tags, Shared with me, Trash, Help, Feedback, Team and Settings.
New note (the green button) makes a note in the project you're looking at. The arrow next to it has more ways to create: Today's note, From a template, Voice note, Meeting notes, Canvas, New notebook and Import notes.
Search (⌘K) jumps to any note, notebook or tag. Press ? to see every keyboard shortcut.