"How they have migrated from RDBMS to Cassandra?"
-> there is no one-answer-fits-all. It all depends on the application features
"What are the things to consider?"
-> design data model with query-first approach. The choice of how/when/what to denormalize is crucial
"How they have converted data model and after the new data model?"
-> for zero downtime migration you'll probably need double-run strategy where you insert into old (RDBMS) and new (Cassandra) data store at the same time but read only hit the RDBMS. Then launch data migration script to copy existing data from RDBMS to Cassandra, then cut all read/write access to RDBMS
"How they have loaded the data into cassadnra ?"
"Performance test after and before migartion etc."