If you use this code, it will upload your file in storage/app folder-
Storage::disk('local')->put('file.txt', 'Contents');
Ref: https://laravel.com/docs/5.8/filesystem
Now, if you need to change the directory and store into the storage folder directly, you need to change something in the filesystems.php file.
Go to config/filesystems.php file and change the following code-
'local' => [
'driver' => 'local',
'root' => storage_path('app'),
],
Here, this line of code 'root' => storage_path('app'), responsible to define where to store. You just adjust according to your demands.