To became a backend engineer this fundamental concept need to know.

• TCP
• UDP
• Web socket
• gRPC

• HTTP
• Caching
• Static, Dynamic
• ETag and ETag generation
• Threading
• Multiple Jobs like Nginx (Web server and proxy)

• LSM
• SQL
• ACID
• NoSQL
• Scalability

• Proxy
• Reverse proxy
• Caching layers
• Load balancers
• TLS termination
• Service mesh

• Stateful caching
• Stateless caching

• Understand concept of messaging system
• e.g. RabbitMQ, Kafka, ZeroMQ etc.

• Architecture of a web framework
• e.g. Nodejs, Django, Express etc.

• Protocol buffers ( minimize payload )
• XML
• JSON

• Encription
• TLS (Transport Layer Security) between messaging and networking
• MitM Attack
• Replay attack
• Database credential
• Firewals
• Denial-of-Service attack (DoS attack)
• Web security, Network security

This will be updated day by day. This is only for my learning prefarence. You can also follow this stapes if you wish.