There are websites where you can put in the details of a YouTuber and it will give you estimates of their income based on their views and level of in video advertising.
There is also sponsorship, Patreon, merchandising etc.
His Ltd company and company accounts are easy enough to find if you go down a rabbit hole of the planning link shared by @JCA
He seems to be doing very well. Good on him. He makes great, engaging, inspiring content. Why shouldn’t he be be rewarded similarly to TV and conventional media personalities.
Writing off the cost of a house in Lincolnshire is probably worth it.