Java, Kotlin, Nfc, Smartcards, Trusted execution environments, Cross-process communication in Android, Hardware integration on Android, Large-scale application design and architecture

S3, Dynamodb, Node.js, Kotlin, Datadog, React, Ec2, Sqs, Splunk, AWS, e-commerce billing, event-driven systems, software engineering fundamentals, REST API design, OAuth 2.0, microservices architecture, SignalFx, observability tooling