A few points which i could recollect from the course An Introduction to SAP HANA by Dr. Vishal Sikka - Dr. Vishal Sikka
Column store is divided as main and delta storage where main will be column store and delta store will be of row store. New data will go into delta store
and they are merged frequently with the main. When there is a look up for values then a join of main and delta will be display. When they are no open transactions the merge process will delete the records from memory.
It is also explained in this blog HANA column store backgrounds (INSERT ONLY)