From MS DOCS on char and varchar (Transact-SQL):
char [ ( n ) ]
Fixed-length, non-Unicode string data. n defines the string length and must be a value from 1 through 8,000. The storage size is n bytes. The ISO synonym for char is character.
varchar [ ( n | max ) ]
Variable-length, non-Unicode string data. n defines the string length and can be a value from 1 through 8,000. max indicates that the maximum storage size is 2^31-1 bytes (2 GB). The storage size is the actual length of the data entered + 2 bytes. The ISO synonyms for varchar are char varying or character varying.
So for varchar, including max - the storage will depend on actual data length, while char is always fixed size even when entire space is not used.