To export from Tropy, right-click on an item in the item table. From the right-click menu, select Export Item > JSON-LD. This action will open a dialog box, where you can save your JSON file onto your computer. To export multiple items, Ctrl/Command+click or Shift+click to select multiple items. Then right-click and select Export Selected Items.
At present, Tropy only exports metadata and notes to JSON-LD, not associated photos. Your photos still exist outside Tropy as well, since importing in Tropy only links to your photos.
You can export your notes in either HTML or Markdown. To change which format you wish to use, navigate to Preferences and scroll down to Export. Select either "Notes as HTML" or "Notes as Markdown."