Every website used to be like “www.example.com”. Now I rarely encounter this “www.”. Why did it even exist and where has it gone? I also used to run into websites with legitimate domains, but “ww1.” instead of “www.”, these were probably phishing sites, but how did this actually work on top of legitimate domains?
It was only necessary if the site was configured that way which most typically were. This was to differentiate which server would handle the request, web, file, mail, etc. I’m pretty sure there was no RFC that stated it was a requirement. Even on the early web it wasn’t uncommon to see a www2 or some other numbered prefix for different versions of a site. The reason you see it less now is because like you said browsers will add it, but also because most sites no longer use it as web is typically the assumed default.