When you add new content to a section it defaults to the bottom of the section. You can move it to a new location within the section it was created in or move it to a new section.
You can also move content using module index.
To access the Question bank:
Editing Quiz questions in Quiz preview does not immediately display the changes to the question or options.
This is being worked on by Moodle. For a much better question editing experience, open the Quiz, select the link to Questions in the contextual navigation menu above the module name. This lists all questions available in the Quiz. Scroll to the relevant question and select the edit (cog) icon to make your changes. Alternatively, you can edit your question in the Question bank.