sharepoint online – Question for Site Collections [Beginner]

I’m beginner in SharePoint. I’m working in SharePoint 2013.

I’m confused about site collections.

When we are getting site collection, we can do it like this e.g.:

SPSite site = new SPSite("localhost:32000");

Doesn’t this mean that we get more Site Collections than one? If I understand, localhost:32000 in this case is url of web application. And I know web application can have more site collections.

And when we want to create web site to web collection, shouldn’t we specify for which site collection are we making web site?

So am I wrong, can anyone explain please this?