In the sidebar, Ctrl-click on Windows/Linux or Cmd-click on macOS selects individual items. Shift-click selects a range.
Drag a sidebar item onto another to move it inside. If several are selected, dragging one moves the selected group together.
Choose Copy, then Paste where you want it. Pick:
Make a copy
Creates an independent version. Changes to one won't affect the other.
Link existing item
Shows the same item in another place too. Changes made in either place stay synced.
Your Passport can live in Personal Documents and also appear in Japan Trip. It's still one Passport.
Duplicate structure
Copies the organization but leaves the content behind. Choose a new name; Notes, Files, Media, Bookmarks and Locations aren't copied.
Mexico trip
→ Hotels
→ Restaurants
→ Transport
Japan trip
→ Hotels
→ Restaurants
→ Transport