What is the difference between somewebsite.com, www.somewebsite.com and somewebsite.com/?

I have 3 questions mainly. There are questions like this in the internet, however I have not found a duplicate question like this in stackoverflow. Anyway, I will appreciate your answer.

1. I know that www.somewebsite.com is a subdomain of somewebsite.com. So I assume it means that hypothetically I can have two different websites in those two different urls although not recommended. Am I missing something here?

2. Between somewebsite.com and somewebsite.com/ if you don't do a 301 redirect, then will somewebsite.com/ show a page directory?

3. Why do big companies like google, amazon etc redirect their domain to the www subdomain?(Legacy, convention, best practice?)

Thanks!