What is a tenant? From this MSDN article:
A SharePoint 2013 tenancy is a set of site collections in either a SharePoint farm or in SharePoint Online. In SharePoint Online, the site collections belong to a single customer account. In a SharePoint farm, the site collections can be all the site collections in a SharePoint web application or a subset of them, or it can be a set of site collections from across multiple web applications in the farm.
It seems that it's still unclear what tenant is, so here is more info
In order to understand what a tenant is, the concept of multitenancy should be explained.
Multitenancy referes to a principal in software architecture where a single instance of the software runs on a server, serving multiple tenants. A tenant is a group of users sharing the same view on the software they use.
From this link, you can see an illustration of a tenant:
Given this information, you can liken a tenant to a Web Application.
Given this, your second and third questions do not make sense.
For your fourth question, yes, Microsoft creates everything on the back end.
You'll need to clarify your fifth question.
From your comment below: what is the scope of the tenant?
There is no scope to a tenant. A tenant is set of site collections, that is all.