Supercell is a Helsinki studio founded in 2010 and one of the most successful mobile developers in history. Its games, including Clash of Clans, Clash Royale, Brawl Stars and Hay Day, are mobile-only and have generated enormous revenue.
The caveat worth flagging is ownership: the Chinese giant Tencent has been Supercell's majority shareholder since 2016. The design and engineering are Finnish, but the ownership is not European.