They invest a great deal of time and energy into key brand elements like tone, brand design and content guidelines. Each one of them has a unique style of speaking and editing their videos. Often, they will come up with fun nicknames for their followers (which further strengthens brand association and sense of community).
Design is also a huge element of their success. Snappy video editing techniques like jump cuts, overlays and title cards add interest to what they’re saying. You’ll see each YouTuber adopt specific iconography, font and colours that become associated with them as their following grows. The key here is consistency.