I have been working my way through the online docs, https://docs.microsoft.com/en-us/sharepoint/dev/. Having completed all the sections under [SharePoint Framework] -> [Web Parts] -> [Getting started] I feel no less confused about how I should approach our design. I have read further forward to [SharePoint development overview] -> [Programming models] and the advice seems to be 'use an Add-in'. We have an Office 365 account so we are using SharePoint Online.
I want to 'code' an App which will allow a logged in user to create a list of date amounts and images (actually expenses) for a chosen month. On completion their manager would be emailed with a link allowing them to authorise or question the amounts/entries. On authorisation another email would be sent to Accounts to allow further processing or request for clarification.
I imagine a file/folder structure of Root\User\Year\Month for each submitted list. The App would request any user not logged in to log in. The user would then select the year and month for the expenses claim. The folder would be created for an new one and the list opened allowing new entries to be created. If a list already exists this would be opened.