The primitives

Learn — load balancing (L4 vs L7), caching layers and invalidation, cache-aside versus write-through, SQL versus NoSQL by access pattern, sharding and hot keys, replication lag and read-after-write, queues and back-pressure, CAP applied to a real outage, idempotency, consistent hashing, CDNs.

Do — for each primitive, write three sentences from memory: what problem it solves, what it costs, and when it is the wrong choice.

Check — you can explain read-after-write to someone who has never heard of replication, without using the word "eventually". You can name a case where adding a cache made a system worse.

Ship — a one-page cheat sheet in your own words. Not a copied diagram.

Doing this first is what stops design rounds from being vague hand-waving.