intersection of polygons and union of polygons

Good afternoon, I have points on a map I want to generate a voronoi diagram on the points, and I want the limit of this voronoi diagram to be a polygon (the city), and once these polygons created would like to unite some between each other, I need tools or methods in C # or js, or even an algorithm only. taking into account that the diagram grows to infinity or a very large grid, how would I pick up the intersection area then the diagram and the polygon, and how would I join two polygons? I also have an oracle space database where I save the polygons.