Check out example codes for "open gallery android". It will help you in understanding the concepts better.

Code Example 1

// Select Image From Gallery
Intent intent = new Intent();
intent.setType("image/*");
intent.setAction(Intent.ACTION_GET_CONTENT);
startActivityForResult(Intent.createChooser(intent, "Select Picture"),SELECT_IMAGE);

// Override this method too
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == SELECT_IMAGE) {
        if (resultCode == Activity.RESULT_OK) {
            if (data != null) {
                try {
                    Bitmap bitmap = MediaStore.Images.Media.getBitmap(getActivity().getContentResolver(), data.getData());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        } else if (resultCode == Activity.RESULT_CANCELED)  {
            Toast.makeText(getActivity(), "Canceled", Toast.LENGTH_SHORT).show();
        }
    }
}

Code Example 2

Intent i = new Intent(Intent.ACTION_PICK,android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
 startActivityForResult(i, RESULT_LOAD_IMAGE);

Learn ReactJs, React Native from akashmittal.com