Start here. This is the direct spoken answer to practice first.
Why this question matters
Explain transaction isolation levels with practical .NET data-access examples and production trade-offs. It matters in real backend work because query shape, indexes, transactions, locking, and data volume can turn harmless-looking code into a production bottleneck. The practical angle is transaction isolation levels, EF Core/SQL behavior, API boundaries, production risks, and practical debugging or design judgment, tied to a concrete production decision.
Transaction isolation controls how one transaction sees data changed by other transactions. Lower isolation usually allows more concurrency but can allow anomalies like dirty reads or non-repeatable reads. Higher isolation gives stronger consistency but can increase blocking or deadlocks. I choose isolation based on what the business operation needs, not by always picking the strongest level.