"A senior developer can develop a whole application from scratch by himself. Senior devs know about design patterns, architecture, automating testing, performance, security, debugging, bug fixing, testing, validation, etc. This doesn't mean they don't have questions or make mistakes, it means they know how to identify problems and where to find the right answers. That's why they are the ones who get assigned highly complicated tasks that require an exhaustive level of knowledge."
Source: https://lightit.io/blog/identify_seniority_level/