How is whatsapp website web.whatsapp.com and not www.whatsapp.com. How can we register a website domain starting with 'web.'
If you own a domain, you can add any subdomains you want to it. It can generally be configured in server settings. If you use a hosting provider, they should have an option to add a subdomain too.
Oh, and the "web" is not exactly a replacement of "www". While "www.example.com" and "example.com" point to the same resource, "subdomain.example.com" will point elsewhere unless you set it to redirect.