they have brand deals, promotions, sponsors and patreon. a asian ones i used to followed had to use these to stay alive they barely make 2million views per video. youtube does all sorts of stuff to your algorithim which can exclude you from reach other peoples feeds or not all. the big ones certain are using these+ they are already wealthy and expands to have thier own company which contributes more.

some are quite small and only upload occasionally and dont sustain themselves on youtube at all, mostly the ones about sciences as many of them are burgeoning scientists, going to school,etc.