It is thought that www and http:// (https://) are independent, both existing in all URLs. Some websites start with http:// while others start with https://, but the latter may be hidden; you don’t need to input it, and it will appear automatically. Therefore, currently, most people can just enter www to access a website, but there are exceptions.
http:// or https:// is a type of protocol. Common URLs will automatically complete in the address bar, so you can just enter www. However, some require you to input http or https manually because those web pages do not have the auto-completion code, requiring manual input to access.
HTTP (HyperText Transfer Protocol) is the most widely used network protocol on the internet. All WWW files must adhere to this standard. What you refer to as starting with www is actually starting with http://www or https://www.
www is the default subdomain of the homepage, while http:// is like an access instruction for visitors entering a building; https:// is the double assurance of that access instruction. This is like driving a completely unmarked black car to the front of the building, being checked by a personal bodyguard for every detail to ensure you are not being followed.
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