There are 9 billion people in the world. I think something like 6 billion watch YouTube, and the biggest creators on YouTube have a hard time breaking 100 million subscribers. That is because all these people have very specific tastes, cultural influences, demographic considerations etc. There is no trick to 'get them all.'
This being the case you need to pick a niche that:
1. You can enjoy making a TON of content around
2. Can support a TON of content.
3. You can be the authority on
Single out your niche, your one little piece of the pie that works best for you. Of that 6 billion people there are hundreds of millions who will respond to the combination of your topical reference and the style/personality that you bring to it. Be mindful of that; your subscribers will be based on human beings who need your topic covered and who enjoy your personality and style to get it from. The selection of niche, subject, topic etc is moot so long as you provide value. Even the most saturated of niches can be broken in to so long as you can provide value to an audience, and the number of people you have an opportunity to get in front of is going to be massive regardless.