Complex sentence: The eastern part of the main island, and a few small islands in the Beagle Channel, belong to Argentina, being part of the Tierra del Fuego, Antarctic Territory and South Atlantic Islands Province; its capital is Ushuaia, the biggest city of the archipelago.
Split into simpler sentences: The eastern part of the main island, and a few small islands in the Beagle Channel, belong to Argentina. They are part of the Tierra del Fuego, Antarctic Territory and South Atlantic Islands Province, whose capital is Ushuaia, the biggest city of the archipelago.