ETL (Extract, Transform, and Load) and ELT (Extract, Load, and Transform) are two paradigms for moving data from one system to another. The main difference between them is that when an ETL approach is used, data is transformed before it is loaded into a destination system. On the other hand, in the case of ELT, any required transformations are done after the data has been written to the destination and are then done inside the destination – often by executing SQL commands.
Yes, transforming the data before it hits the destination may be necessary in some cases, even in the ELT approach (hence the "t" / "tweak") - discussed here: https://airbyte.com/blog/etlt-gdpr-compliance