This can be a little easier said than done, but if you have a large social media following, you can definitely earn money promoting a product or advertising for a company. You can even combine this with different marketing campaigns if you are an influencer and have your own blog (advertisement + affiliate income). This is how many bloggers make money! Again, it is not 100% passive but once set up correctly and then scaled, can be surprisingly lucrative.
That income is considered residual income because as long as the apartment is rented and the rent is collected, the income is earned without additional effort. The effort came when the property was purchased and a tenant was found. Each month after that, the money automatically is paid without buying the apartment again or finding the same tenant each month.
Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.