To migrate data from Cassandra or MySQL to YugabyteDB, users can utilize the CSV format as the most universally portable way to accomplish data migrations. YugabyteDB uses Cassandra's COPY FROM command and a forked version of Cassandra's Bulk Loader for importing data and Cassandra Unloader for exporting data into a CSV format. The process involves creating a destination table in YugabyteDB, exporting data from Cassandra or MySQL, and then importing the data into YugabyteDB using the recommended tools and methods based on the size of the dataset. Additionally, users can export data from YugabyteDB into CSV files using the cassandra-unloader tool.