Can Netlify protect whois information? I mean someone can not read my info from whois?
As per Netlify’s Domain registration docs:
All Netlify domain registrations are private, with Netlify listed as the WHOIS contact. Netlify-registered domains are automatically configured to use Netlify DNS.
can I use Cpanel or phpmyadmin or wp-admin or something like that after I buy a custom domain on Netlity? or can I use another custom domain supplied by another Host?
Yes, you can Configure external DNS for a custom domain.
How can I upgrade/update my custom website with Netlify uninterruptedly?
That depends on what you have used, or are using, to build and maintain your website.
ok, thanks and the price of Netlify’s domain gives me including the price of the host?
Netlify platform pricing is available on the pricing page. Cost of domain registration is separate.