Choosing between collections, Embedded database and centralized database

I am a beginner so please be tolerant while reading the question. I tried to search the web for answer but didn't find anything useful. This Answer is incomplete I am building an app for employee management. I am using visual studio, C# and UWP (Universal Windows Platform) for the purpose. The app will deal with salaries, leave and personal details of employees. That amounts to a lot of data, especially in long term. The app will serve home based businesses to large enterprises. The app will save data locally and sync with cloud based database. In the near future the database will serve and get data from a website and client apps. The clients will include ios, android, windows phone, pc, mac and linux. So far i have worked with collections to handle data in apps and have basic knowledge of SQL. The questions is as follows:

1)How to decide (for the sake of any application developer) when to use collections, when to use embedded database and when to use RDBMS. when Non-relational databse like nosql or hadoop is applicable.

2) Different options for local database.

3)Different options for cloud database .

4) Can sqlite work on cloud to serve millions of users.