How exactly do you make money online?

The most mainstream one is YouTube or Twitch I'd say, definitely no the best and don't suit everyone.

Creating asstes for gaming industries, like Epic Marketplace or Unity. It can range from 2D graphics for HUDs to 3D models for characters and environments, sound effects and dialogues are an option as well. (Not to mention coding, for game bases or components).
Photography services, such as editing photos or even organizing shoots.
Video editors tend to be popular as well, even tho it might require you more trasversal skills, not only limited to that.
Selling physical objects that you've crafted or some sort of very appealing merch you can create online.
Art commissions, like custom drawings or animated sequences are doing well.
Invest in some stuff and sell them for higher prices in aces lime ebay or other second hand online stores in your country.
Make courses about something you excel at, it could be really anything from playing an instrument to bussiness strategies, Udemy and Skillshare are the most popular communities for doing that.

You can do whatever basically as long as you put in effort and make it your main focus, it will take time, patience and resources but if you're good at it and believe in yourself you can succeed indeed.
Lot's of people do, I do not but all these examples are from what I've seen people do.