The organizing principle behind this grouping, appropriate economic units, is relatively simple: if the activities are located in the same geographic area; if the activities have similarities in the types of business; or if the activities are somehow interdependent, for instance, if they have the same customers, employees or use a single set of books for accounting.
The other point is that it is pretty easy to get started. You don’t need to be super rich, and you don’t need a lot of time to get started. To say it requires no time would be a lie, but you don’t need to make anything listed above your life. You can work at your job, invest your excess income, save to buy a rental property or rent out a room in your current house, and you start a side job online without breaking a sweat.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.