Your machine is not powerful enough to make anything other than very basic setups, but you can always learn some things by installing a Linux distribution, try to learn how to use the command line, maybe install SSH and learn how to connect to your machine remotely.
If you have never touched a linux command line and played with a Linux server, self-hosting your email server will be too complex and you should start there (by getting a cheap VPS for example).
But if you have a server, a domain name, and some knowledge about self-hosting, there are some easy and FOSS “all in one” solutions like mailcow (https://mailcow.github.io/mailcow-docs/) that bundles all the required software in docker and gives you an admin panel to create mailboxes. It also tells you what DNS records to create and checks if you’ve done it right.
It’s not as hard to manage an email server as many say, just make regular backups to restore if you do break the setup, and don’t worry too much about downtime, generally other servers will keep retrying to send you your emails for quite some time (hours to days) before giving up.
The company owns the “example.com” domain name and can put what they want before it. The “www.” part (which stands for World Wide Web) was a convention for the company to put their website on, as other subdomains may be used for other services than websites. As time went on, companies started to make the “example.com” itself point to the website as it is shorter and easier to type/remember, and made the “www.example.com” redirect you to “example.com” for retrocompatibility