You can select any of the above-mentioned, based on your interest, skill, and capability to generate a second income source. However, these are just to name a few, there exist multiple ways to generate a secondary income channel. You just need to identify the right one, which suits you the best. Remember there is no shortcut to success and you need to work hard to be successful and rich in the long run!
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.