Understanding SharePoint URL's

My application receives the SharePoint URL from where it is invoked.

However, the URL contains additional information (which I do not need). For example my library URL is as follows:

https://contoso.com/subsite/library

However, teh URL in the browser (which my app retrieves) looks like this:

https://contoso.com/subsite/_Layouts/15/start.aspx#library/Forms/AllItems.aspx

Questions:

1) Is there somwhere a description how this URL is built up? e.g. what _Layouts/15 etc. means, what variations there are etc?

2) Is there a function that I can either retrieve the Library URL (https://contoso.com/subsite/library) or a function that converts the URL in the browser to the library URL?