Specifically, in context with the current Hungarian community there today. How did it get there, and how big was it at its peak. Were Romanians a majority endemic to Transylvania prior to the Magyar invasion, or was Romanian domination of Transylvania ethnically a later development in the Ottoman wars?

Basically, why does Transylvania have an isolated Hungarian community, and what was relationship between these two peoples during the community's formation and existence.

Feel free to only answer one question, because this probably a very complex answer.