I don't understand. How is it not a node if the finalGContainer was succesfully created using the "createElement" function?

as pointed out in the comments previously, createElement does not create a DOM node. When you console log finalGContainer you'll see it's a Symbol instead of a <g> SVG element

In order append finalGContainer as a child of the SVG, you have two options

• set the SVG as your root react element, then root.render(finalGContainer)
• convert your react code to JavaScript (also previously recommended) by
  • converting finalGContainer to a string of HTML
  • parsing that string of HTML into actual HTML
  • attaching the parsed HTML's first child ( just the HTML of finalGContainer) to your svg

const htmlStr = ReactDOMServer.renderToString(finalGContainer)
const domNode = new DOMParser().parseFromString(htmlStr, 'text/html').body.firstChild
svg.appendChild(domNode)