It is unclear what methodology RT has used to reach the claim that it is the most-watched TV channel on Youtube. Anything close to the 10 billion figure can only be achieved by adding up viewership of all RT services in all languages. If we apply the same methodology to the BBC and CNN, they will comfortably pass the 10 billion views mark. As of 12/02/2020, the CNN News network has reached, at the very least, 11 billion views, and this does not include the non-news services owned by the CNN such as HBO. As for the BBC, the Youtube views of BBC news services in only 14 languages (out of 27) were at least 14 billion in 12/02/2020. This also does not include content such as sports and children's entertainment. It is also not clear whether RT has included its non-news outlets/Youtube channels (e.g. RT Sport) to the 10 billion figure.
See a similar earlier case here.