Hi @stadi :
The structure of the leave request template is very complex, involving a large number of custom variables and collections, so it is difficult to clear it out in a few words.
Let me briefly introduce you to the design logic of this app:
1\Most collections and variables are defined by OnStart event.
Some types of leave requests, description of the leave request, current user data, email format, vacation
The leave form template (you can replace it with your own data source), etc. are loaded when the application is loaded

After you understand these variables, your understanding of the entire app will be deeper.
2\You can choose different roles to enter HomeScreen. App uses the OnVisible event of HomeScreen to determine the user's role and perform different operations.

3\After the user fills in the leave application, he will jump to the ConfirmationScreen. The app submits the user's leave application through the OnVisible event of ConfirmationScreen.

4\If the current user is a manager, submit the approval information through the BtnManagerApprove button.

In addition,If you just want to make some simple customization based on the template, you can refer to the following link:
https://www.youtube.com/watch?v=Skb1Y9YujYA
https://www.youtube.com/watch?v=_ECN8QWiNrQ
https://www.youtube.com/watch?v=U2bbu8IDCK8
If you want to build a similar app from scratch, you can focus on the important nodes I mentioned.
Best Regards,
Bof