I had opened and edited Alloy site (example site) but can I edit any site made with Episerver as the same way as Alloy site?
Yes the edit and admin interface is what you get with the CMS product from EPiServer. (That and a well built API to code against instead of having to have your own database access)
But by default all websites running EPiServer will have the same looking Edit and Admin mode.
Hi, Does it mean that it also works with a DEMO License or only with Developer or higher license?
Both Demo and Developer license will work to show you the edit mode.
(Infact you could run with out a license at all. But if you do you will get a notice about running without a license)
Demo licenses will last for 1 month and a Developer license will last for 1 year.