I’m not sure I understand exactly what you’re trying to do? In the project view, you can switch between seeing all items and tagged items or items in a list by selecting the respective lists/tags in the sidebar or by clicking on the project name to clear the selection.
When you select items in the project view, the photo panel will show all the photos of these items. When you open an item, Tropy switches to the item view where you can only see the photos of a single item.
I guess your question is about switching back and forth between item and project view? You can press escape (or the back button on your mouse, if it has one) to switch back to project view while you’re in item view. In item view, the current item should still be selected so you can often just use the keyboard shortcut to go up or down a few items and open that one to switch back to item view.
If you would like to switch items, while in item view, without going back to the project view, you can do so using global short cuts: Alt + Arrow up/down moves to the next or previous item (Alt + Arrow left/right moves to the next or previous photo).