The + button in the app header always creates a page. Use it if you want a page while a chat is open.
Page undo includes text edits, block deletion, todo checkboxes, and Move to History. It applies to the page you last worked in, even after clicking a checkbox or archive control. Text fields such as the chat composer keep their own undo history.
While writing
Type/ on an empty line to open the insert menu. Search for the block you need, such as a heading or to-do list.