There are multiple methods that many of the now-big YouTube channels have used in the past and present to boost their channels. Stuff like:
- collabs between channels to expose their audience to eachother's content
- Posting and promoting their videos outside of YouTube (Reddit, Forums, Blogs, Etc.)
- Viral Content (This is a big one, but also the hardest to truly do)
- Paid Promotion
- Etc.
The main trick is getting exposure. A lot of the time, people coming into YouTube who want to make money with it don't understand what really goes into it. The way most of these big channels stay big is that they actually enjoy what they do and don't just churn out content for the sake of doing so. That and people also tend to just upload and pray it gets attention. Sure you might get a few views here and there but that's not an efficient way to get it noticed. If you create good content that people like and expose it to enough people (i.e. Posting it on a related subreddit and using the algorithm in your favor to try and get it to the front page, or even onto r/all), it will start snowballing.