If your main source of income is making YouTube videos, then yes, multiple streams of income are a necessity. YouTube’s algorithm regularly screws over creators in new and interesting ways, so you never know if you are going to make money that month or not.
For people who have regular jobs, you typically need a full time job so that you can get decent health insurance. A second job will take up all of your free time and will often run you ragged. Sure, you will have more money, and if one job goes belly up you’ll still have income from the other. But is that really the life that you want to live? You won’t have time to do anything fun.
If your primary job pays well enough, then you have some choices. You can choose to live in a fancy house and drive a fancy car, or you can choose to live in a more modest house and drive a much less expensive car, freeing up a bunch of money that you can do something with. You can invest in the stock market, bonds, commodities, etc. You can take a chance and invest in some startup company. Some of these investments (especially a startup company) are rather high in risk, and in general, the higher the risk, the higher the reward if you succeed. And of course the higher the probability of complete failure, which could lose your entire investment.
Another option is to figure out how to monetize something that you enjoy doing. Your example of a teacher writing a textbook is a good example, assuming that the teacher actually enjoys writing textbooks. If the teacher doesn’t enjoy writing textbooks, then they are just setting themselves up for hours of misery that will probably not give them a lot of income. I know someone who enjoys drawing and painting. She has a regular job, and also has a side business drawing and painting people’s pets. She makes a decent amount of money doing it. I have a cousin who used to like tinkering on cars. He’d buy an old crappy car, spend several months fixing it up and making it nice, then he’d sell it. He made a profit on every car, but he wasn’t doing it for the money. He just enjoyed working on cars.
I personally chose to invest in a couple of rental properties. I’m happy with it, but I don’t recommend it for most people. If you think you just buy the property and then just sit back and enjoy the money coming in, that’s not the way it works. Not even close. It also requires a lot of money to get into it. You need a much higher down payment for a rental property than you do for your personal home that you are going to be living in. Around here, you typically need 1/3rd of the price of the home as a down payment for a rental property, plus closing costs, etc. You also need to have enough money set aside that you can handle any major repairs that might come up, or let’s say something really weird happens like a global pandemic, and all of a sudden your tenants are having trouble paying their rent. If you can’t go at least six months without receiving any rent at all, then buying a rental property probably isn’t for you.