SQL and complex SOQL (salesforce object query language). Knowledge of Batch classes, chaining batch classes, queueable apex classes.
Custom Metadata Types, Custom labels, and Custom Settings and when to use each
For an API Developer: Difference between asynchronous and synchronous, Salesforce Exposing Rest Resource (aka creating an endpoint in salesforce), making Api callouts from salesforce with Named Credentials, creating Connected App
Flows: Screen Flow, Record Triggered Flow, when to use fast field update or Before Save flow and After Save type of flow
Permission Set and Profile, when to use each; Debugging and using Trace Debug