I was struggling to find the right way to set a default font in React Native 0.72.x.

This is what I ended up with eventually:

const defaultTextStyle = {
  fontSize: 16,
  fontFamily: Constants.fontFamily,
};

const originalRender = Text.render;
Text.render = function render(props) {
  props = {
    ...props,
    style: [
      defaultTextStyle,
      props.style,
    ],
  };

  return originalRender.apply(this, [props]);
};