ReactDOMFiberComponent

https://github.com/facebook/react/blob/master/packages/react-dom/src/client/ReactDOMFiberComponent.js#L333

createElement

export function createElement(
  type: string,
  props: Object,
  rootContainerElement: Element | Document,
  parentNamespace: string,
): Element {
  let isCustomComponentTag;

  // We create tags in the namespace of their parent container, except HTML
  // tags get no namespace.
  const ownerDocument: Document = getOwnerDocumentFromRootContainer(
    rootContainerElement,
  );
  let domElement: Element;
  let namespaceURI = parentNamespace;
  if (namespaceURI === HTML_NAMESPACE) {
    namespaceURI = getIntrinsicNamespace(type);
  }
  if (namespaceURI === HTML_NAMESPACE) {

    if (type === 'script') {
      // Create the script via .innerHTML so its "parser-inserted" flag is
      // set to true and it does not execute
      const div = ownerDocument.createElement('div');
      div.innerHTML = '<script><' + '/script>'; // eslint-disable-line
      // This is guaranteed to yield a script element.
      const firstChild = ((div.firstChild: any): HTMLScriptElement);
      domElement = div.removeChild(firstChild);
    } else if (typeof props.is === 'string') {
      // $FlowIssue `createElement` should be updated for Web Components
      domElement = ownerDocument.createElement(type, {is: props.is});
    } else {
      // Separate else branch instead of using `props.is || undefined` above because of a Firefox bug.
      // See discussion in https://github.com/facebook/react/pull/6896
      // and discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1276240
      domElement = ownerDocument.createElement(type);
    }
  } else {
    domElement = ownerDocument.createElementNS(namespaceURI, type);
  }

  return domElement;
}

getOwnerDocumentFromRootContainer

https://github.com/facebook/react/blob/master/packages/react-dom/src/client/ReactDOMFiberComponent.js#L219

function getOwnerDocumentFromRootContainer(
  rootContainerElement: Element | Document,
): Document {
  return rootContainerElement.nodeType === DOCUMENT_NODE
    ? (rootContainerElement: any)
    : rootContainerElement.ownerDocument;
}