encountered this pit and has been resolved. https://zhuanlan.zhihu.com/p/.
ios directly use CameraRoll

android:

//saveBase64ImageToCameraRoll.js
import RNFetchBlob from 'rn-fetch-blob';
import RNFS from 'react-native-fs';
import { Alert, Platform, CameraRoll } from 'react-native';

export default function (base64Img, success, fail) {
  
  const dirs =  Platform.OS === 'ios' ? RNFS.LibraryDirectoryPath : RNFS.ExternalDirectoryPath; // android
  const downloadDest = `${dirs}/${((Math.random() * 10000000) | 0)}.png`;
  const imageDatas = base64Img.split('data:image/png;base64,');
  const imageData = imageDatas[1];

  RNFetchBlob.fs.writeFile(downloadDest, imageData, 'base64').then((rst) => {
    try {
      CameraRoll.saveToCameraRoll(downloadDest).then((e1) => {
        console.log('suc',e1)
        success && success()
      }).catch((e2) => {
        console.log('fai',e2)
        Alert.alert('[]-[]-[]')
      })
    } catch (e3) {
      // Alert.alert(JSON.stringify(e3))
      console.log('catch',e3)
      fail && fail()
    }
  });
}

found that higher versions of android will flicker when using this
try to use RNFS, but there will be a problem, and it is still being resolved
July 13

just used it yesterday, just save it directly, that is, use the CameraRoll to find an article on the Internet to learn how to configure it. It's not difficult.