Video narration
What happens after you press Execute in SQL Server Management Studio and wait for rows? Your query reaches the relational engine, which understands SQL and chooses a plan based on the query and available data. That plan passes to the storage engine, which finds or changes the data pages needed. Here's the flip: SQL Server doesn't simply fetch every answer from disk. The buffer pool holds data pages in memory, so a read can be served without going back to files. If the needed page isn't in memory, SQL Server reads it from the database files. For changes, the transaction log records what changed, while database files remain the durable store. So what happened? The relational engine planned your query; the storage engine handled it, using memory or files before returning results.