It’s upto the youtuber how they allow the public to contact them.
Here are some steps :
1. Become a member of youtube.
2. Click on the inbox.
3. Click compose.
4. Write the messsage like you would an email.
5. send it.
But if you want to be famous in youtube follow the steps >
1. Chose a descriptive username that is easy to remember.
2. Optimize the Channel Page with an original, interactive and attractive design to best represent YOU.
3. Include links to additional locations & Social Networks to supply navigation to the viewer to learn more about YOU.
4. Include all possible information including contact info (personal email & phone numbers), mailbox address (for product reviews), Agency & Business Inquiry information, and links as to where viewers can find merchandise to buy (if applicable).
5. Plan a video tone/ technique and release schedule.
6. Create a brand persona, similar to your own, that is compelling & interesting for viewers.
7. Create merchandise to be able to sell on affiliate websites and your own site.