If i wanted to make my region like a county for example in the USA, how can I make just one courthouse for the entire region not just in every city within the region? This includes other services associated with a county not just courthouses.
Regional play doesn't quite work like that, sadly. Community services cannot extend past a city's borders. You do have options for moving electricity, water, and garbage across tiles, as well as commuters, of course.
There is a solid amount of custom content available to make it almost seem this way though. Things like civics with multiple purposes, or schools that serve elementary and high school students. Civics with wider radii can also help give this county feeling, at least for tiles with small to medium populations.
Yes I know about utilities and commuters just wish other stuff would go regionwide oh well.