Make a state for loading, set it to true, and then show a loading element if the state is set to true. When your API loads, change the loading state to false.

class App extends React.Component {
  state = {
    loading: true
  };

  api() {
    // axios API call here
    // ideally you would callback
    // but for simple terms:
    this.setState({
        loading: false
    });
  }

  componentDidMount() {
    setTimeout(() => this.setState({ loading: false }), 1500); // simulates an async action, and hides the spinner
  }

  render() {
    const { loading } = this.state;

    if(loading) { // if your component doesn't have to wait for an async action, remove this block 
      return null; // render null when app is not ready
    }

    return (
      <div>I'm the app</div>
    ); 
  }
}

ReactDOM.render(
  <App />,
  document.getElementById('app')
);

Or you could make the loader using the CSS :empty pseudo class to show a loader when the list is empty (before your API fills it):

.loader:empty {
  position: absolute;
  top: calc(50% - 4em);
  left: calc(50% - 4em);
  width: 6em;
  height: 6em;
  border: 1.1em solid rgba(0, 0, 0, 0.2);
  border-left: 1.1em solid #000000;
  border-radius: 50%;
  animation: load8 1.1s infinite linear;
}

@keyframes load8 {
  0% {
    transform: rotate(0deg);
  }
  100% {
    transform: rotate(360deg);
  }
}

Or you can use both techniques to ensure the loader always shows, and there's no JS-like FOUC. Shoutout to Ori on StackOverflow for the snippets.