I only know the svg drawing program Inkscape, and in that you select Edit/XML editor.
In that certain nodes/objects have to be certain groups
PCB XML group structure.
silkscreen - non important lines/text that you want in the silkscreen
inside copper1 is copper0 - contacts and holes. Copper 1 is top copper layer on a 2 sided board and copper0 is bottom layer. Because Copper0 is inside Copper1 everything gets duplicated on top and bottom.
You are basically editing the XML file without having to know XML if you use the XML editor in Inkscape.
Make sure dimensions are correct in the PCB svg because it's for production. It practically has to be a simple drawing engineering drawing.