How does Portal work?

export function Portal<T extends boolean = false, S extends boolean = false>(props: {
  mount?: Node;
  useShadow?: T;
  isSVG?: S;
  ref?:
    | (S extends true ? SVGGElement : HTMLDivElement)
    | ((
        el: (T extends true ? { readonly shadowRoot: ShadowRoot } : {}) &
          (S extends true ? SVGGElement : HTMLDivElement)
      ) => void);
  children: JSX.Element;
}) {
  const { useShadow } = props,
    marker = document.createTextNode(""),
    mount = () => props.mount || document.body,
    owner = getOwner();
  let content: undefined | (() => JSX.Element);
  let hydrating = !!sharedConfig.context;

  createEffect(
    () => {
      // basically we backdoor into a sort of renderEffect here
      if (hydrating) (getOwner() as any).user = hydrating = false;
      content || (content = runWithOwner(owner, () => createMemo(() => props.children)));
      const el = mount();
      if (el instanceof HTMLHeadElement) {
        const [clean, setClean] = createSignal(false);
        const cleanup = () => setClean(true);
        createRoot(dispose => insert(el, () => (!clean() ? content!() : dispose()), null));
        onCleanup(cleanup);
      } else {
        const container = createElement(props.isSVG ? "g" : "div", props.isSVG),
          renderRoot =
            useShadow && container.attachShadow
              ? container.attachShadow({ mode: "open" })
              : container;

        Object.defineProperty(container, "_$host", {
          get() {
            return marker.parentNode;
          },
          configurable: true
        });
        insert(renderRoot, content);
        el.appendChild(container);
        props.ref && (props as any).ref(container);
        onCleanup(() => el.removeChild(container));
      }
    },
    undefined,
    { render: !hydrating }
  );
  return marker;
}