What happens to my files and pictures (more than the free 5GB) if I cancel my subscription?

See the article Archive or make copies of the information you store in iCloud - Apple Support

if free 5GB is already exceeded the files and photos will not stay on iCloud , read Manage your iCloud storage - Apple Support

As per this article Downgrade or cancel your iCloud storage plan - Apple Support

If you downgrade or cancel your plan, the storage change takes effect after your current subscription billing period ends.* If you downgrade to the free iCloud Storage plan, you'll have 5GB of storage.