Some good writing here! I am a realtor myself and frequently get in touch with clients that consider buying a realty estate a conservative of investing. I once heard of a transport company in Vienna, Austria, which focused their entire profit on buying eventually every house available in the downtown for about 80 years. That must be some of a passive income!

Active income is needed because you know you can always push away to bring in steady income. Passive income is needed to bring in a little extra on the side. You must ensure to never put all your eggs in one basket. When generating multiple streams of income, you must have different sources to rely on – because in the end, nothing is 100% reliable.
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.