Why does the website URL begin with
http://www1 rather than
It happens that they've enabled wildcard DNS, and will serve up requests to subdomains named, well anything at all. No, really. Anything at all.
What, you doubted?
Long, long ago, serving up the same content on multiple subdomains named "www#" was a cheap and simple load balancing technique. Each domain would correspond to a physical server. It fell out of common practice with the advent of sane reverse proxies and simplified virtual hosting technologies.
www and www1 are just subdomains of w3schools.com. There is no difference between a www and a www1 subdomain. You are wondering why it's www1 since typically the web uses www, but it makes no difference. It could be abc.w3schools.com with the same results. Determining the actual reason w3schools used www1 there would be a waste of time.