Complex sentence: There are no gates in the main Terminal building; instead there are three separate oblong Satellite buildings in which the gates are located, with a fourth satellite building planned, in addition to an extension to the main terminal building, but never completed.
Split into simpler sentences: There are no gates in the main Terminal building; instead there are three separate oblong satellite buildings in which the gates are located. A fourth satellite building was planned, in addition to an extension to the main terminal building, but never completed.