Columnstore tables and indexes are always stored with columnstore compression. You can further reduce the size of columnstore data by configuring an additional compression called archival compression.
Note: Columnstore - The columnstore index is also logically organized as a table with rows and columns, but the data is physically stored in a column-wise data format.
Reference:
https://docs.microsoft.com/en-us/sql/relational-databases/data-compression/data-compression