You need to import 300,000 contacts from a legacy system and relate them to existing accounts. How do you approach it?

Because the volume exceeds the Data Import Wizard limit, I would use Data Loader or a similar tool. I would clean and deduplicate the source data first, and store the legacy system's IDs in External ID fields on Account and Contact so I can relate records and rerun loads safely using upsert.

I would test a small sample in a sandbox, consider temporarily adjusting automation that would slow the load or send unwanted emails, and then validate record counts and sample records afterwards.

Interviewers want tool selection, External IDs, data cleansing, testing and post-load validation. A plus is mentioning load order (parents before children) and the impact of automation and sharing recalculation. A pitfall is loading straight into production without a test run.