So if I upgrade to 50GB iCloud storage plan,what happens when I cancel the plan? Do I lose all the things stored on that 50GB?

Yes. According to: Downgrade your iCloud storage - Apple Support

"If you downgrade or cancel your plan and your stored iCloud content exceeds your available storage, some of your data will not be uploaded into iCloud. Learn how to manage your storage by deleting information that you don't need anymore."