Why some websites doesnt open without WWW but some open with WWW like https://developer.apple.com https://developer.mastercard.com
Few reasons i read are like this states that all traffic is routed though their primary website, does it mean there is no way to access a website through their www.something.website.com form?
www is a subdomain
For any website to open with www the website owner / domain owner has to configure it so with the domain name service (DNS) provider.
As it is a very common and traditionally used subdomain most websites have it configured. But it is not compulsory to have www as the subdomain. Websites can function just normal without it.