Some range hoods need to be vented outside and some range hoods do not need to be vented outside.
There are vented range hoods and non vented range hoods.
The non vented range hoods do not need to be vented outside and use an air re circulation process to vent the range hood.
However the vented range hoods do need to be vented outside so that the cooking smells, smoke etc can leave the house and go outside.
I have the vented range hood which I prefer because the vented range hoods sucks up any cooking smells and even some smoke when I might accidentally burn food which has happened to me before.