How the workflow works in custom fiori like UI5 approval app?

Hi Experts,

I have some questions Regarding SAP UI5 custom app, can you please share your ideas.

     My requirement is to create approval process (2 level approval) in Plant maintenance (EAM), i have searched for standard fiori app it is not available in standard fiori apps so i have to create a custom app for the above functionality...

Now my question are

1. I have to create a custom fiori app or UI5 app, which means i have to create SAP FIORI like UI5 app or is there any separate predefined Templates for creating CUSTOM FIORI apps, if available where can i get those Templates ?
2. Instead of creating Custom app can i use PO approval or Approve Leave Request app for my requirement, if yes what about oData service i have to create a custom oData service or can i change those Standard oData service?
3. When displaying the data to a manager according to manager(using login id) what are the approvals are triggered for that particular manager should only display other managers data should not display, how can i approach this scenario can you please initiate & elaborate ?   
4. For approval process how the Workflow triggers (i have to code anything in controller part to trigger workflow or it will trigger workflow directly when i updates table field status from NEW to Approve or Reject), is there any separate Workflow for FIORI or i have to use SAP Business Workflow only ?

Thank you Everyone for your answers and suggestions.

Thanks & Regards,

Santhosh Reddy.