Convert the CSV files to a self-describing data format, such as AVRO, before loading the data to BigQuery using Data Fusion.

Load the CSV files into a staging table with the desired schema, perform the transformations with SQL, and then write the results to the final destination table.

Create a table with the desired schema, load the CSV files into the table, and perform the transformations in place using SQL.

Use Data Fusion to transform the data before loading it into BigQuery.