Genetically the main group is Germanic (like e.g. in Germany and the Netherlands), but already 1.000 years ago groups of Slavonic people also settled in parts of Denmark and Sweden facing towards present day Poland (and North-eastern Germany). To the very North of the Scandinavian peninsula approx. 100.000 Sami people live - they have most likely been there longer than the Germanic Scandinavians, but nobody knows for sure.
Some pople - especially outsiders - sometimes include Finland in their definition of Scandinavia (which makes perfect sense from a cultural, historical and political perspective, but never-the-less is not usually done by the 'true' Scandinavians themselves, who would use the term Nordic countries when including Finland). The Finns are not a Germanic people.
there's no president of Scandinavia ^^
Norway, Sweden, Denmark, Iceland, Finland
Scandinavia is a region kind of, not a country, so it does not have a capital city. It consists of the countries of Denmark, Norway, Sweden and Finland but also Iceland! Each of those have their own capital citues. Denmark's is Copenhagen. Norway's is Oslo. Sweden's is Stockholm. Finland's is Helsinki. Iceland's is Reykjavik.