Hi gidon,
I am not from Laminar Research; just a flight simmer from downunder.
Let me put you straight from the beginning.
I don't use or support steam. Steam is a 3rd party hybrid taking a commercial advantage of the development work carried by Laminar Research. Laminar does not sell the steam version. If you ask about steam on the Laminar web site you are directed away to another site. There is no commercial connection between either party. Steam installs differently to the official Laminar version of x-plane. Steam does not mix and match with the official version from Laminar. The reverse also occurs.
In relation to your steam question about "extensions' you need to ask steam directly. However if you are asking about updates you will automatically get them from Laminar. Even Beta versions if you want to dabbles in the update/testing process. Once you purchase the official Laminar version you will receive them automatically without any restrictions (number of extensions) at no cost until the version changes from 11.99 to 12.0. The current version is 11.32 Steam does not give you the benefit of immediate updates. They only make it available days after Laminar makes an official release ie when all testing has been completed for that version.
Generally with a steam version you will get no support on this forum. You need to go to other forums that do support steam
Now, to answer your first question. The demo version downloaded from Laminar Research is the same as the full version. The demo restricts you to a set location and has a 10 minute time use once you have selected your aircraft and flight parameters.
Once you officially purchase X-Plane you will be given a 24 digit product key/code. This "key" is applied when prompted during the "boot-up" stage. If you fail to respond the next question will ask if you have a disc version. The disc version requires you to have DVD 1 in a disc drive at all times. If you respond that you do not have a disc version it will automatically revert to the demo version. In short, download the demo version, then purchase the product and then apply the product code. The demo version will open up as the full version.
I would like to think you want to support the developer and not a hybrid.
Glenn