Low-level design.

the two-minute refresher

• LLD answers: how does each piece work internally?
• Typical content: tables, columns and indexes; endpoints and request/response formats; classes and method signatures; core algorithms.
• It also pins down error handling (retries, fallbacks) and in-memory structures such as cache key formats.
• In an interview it is the last 15-20 minutes, on one or two critical components.