char vs byte

e.g.
SQL> 
CREATE TABLE exp_table
(
   col1   CHAR (1 CHAR),
   col2   CHAR (1 BYTE),
   col3   CHAR (1)
)
-> col1 can store 4 byte at most (if db charset is AL32UTF8)
-> col1 can store 3 byte at most (if db charset is UTF8)
-> col3 stores string according to NLS_LENGTH_SEMANTICS parameter
SQL>
SELECT * FROM  v$nls_parameters WHERE parameter = 'NLS_LENGTH_SEMANTICS';

Check db charset
SQL>
SELECT * FROM nls_database_parameters WHERE parameter like '%SET%';

-> NLS_CHARACTERSET set for CHAR, VARCHAR2 and CLOB data types
-> NLS_NCHAR_CHARACTERSET set for NCHAR, NVARCHAR2 and NCLOB (N, National) data types

e.g.
NLS_NCHAR_CHARACTERSET         UTF8
NLS_CHARACTERSET               WE8ISO8859P9

Ref: https://docs.oracle.com/cd/B19306_01/server.102/b14225/ch6unicode.htm