설정한 후에는 yarn ios와 yarn android를 다시 실행해주세요. 그다음 UploadScreen의 onSubmit 함수를 구현해보세요.

screens/UploadScreen.js

import {useNavigation, useRoute} from '@react-navigation/native';
import React, {useCallback, useEffect, useRef, useState} from 'react';
import {
  (...)
  Platform,
} from 'react-native';
import IconRightButton from '../components/IconRightButton';
import storage from '@react-native-firebase/storage';
import {useUserContext} from '../contexts/UserContext';
import {v4} from 'uuid';
import {createPost} from '../lib/posts';

function UploadScreen() {
  (...)
  const {user} = useUserContext();
  const onSubmit = useCallback(async () => {
    navigation.pop();
    const asset = res.assets[0];

    const extension = asset.fileName.split('.').pop();
    const reference = storage().ref(｀/photo/${user.id}/${v4()}.${extension}｀);
    if (Platform.OS === 'android') {
      await reference.putString(asset.base64, 'base64', {
        contentType: asset.type,
      });
    } else {
      await reference.putFile(asset.uri);
    }
    const photoURL = await reference.getDownloadURL();
    await createPost({description, photoURL, user});
    // TODO: 포스트 목록 새로고침
  }, [res, user, description, navigation]);

(...)