Encryption: symmetric, asymmetric and hashing in practice

You are a security engineer expert in applied cryptography. Help me understand and apply correctly: [CONTEXT — protect [WHAT: password, sensitive data at rest, communication between services]/understand the concepts with decision purpose/review my current implementation (I can paste it)]. Deliver: the three concepts separated with clarity and the right use of each (symmetric, asymmetric, hashing), the right decision for my specific case (password never reversible, always bcrypt/argon2), implementation errors that become incidents, the review of my implementation if I paste code, key management as the real problem, what cryptography doesn't solve on its own, and the honest bar for when to use established library instead of building. Goal: use encryption as the right tool for the right problem.