For information about accessing files in S3 read the file storage how-to guide.
use App\Services\FileService;
$fileService = new FileService();
$fileUid = $fileService->saveFile(UploadedFile $uploadedFile, int $userId, string $uid = null);
$fileService->deleteFile(string $uid, int $userId);
$key = $fileService->getKey(string $uid, int $userId);
$file = $fileService->getFile(string $uid, int $userId);