Junior software engineers are typically new grads with less than 3 years of working experience. They typically map to level 3 engineers at FAANG. Without much experience they normally work on a component of a system with help of more senior engineers.
Many companies do not have system design rounds for junior engineers. Some companies however ask OOD or API design questions. Be sure you are familiar with OOP and some common design patterns.
With a few more years of experience than junior engineers, mid-level software engineers usually are expected to be familiar with component design, including how the component interacts with other parts of a system. Questions asked could be data modeling, API design and component design of a product or backend system.
Mid level engineers usually do not have experience building complex systems end to end. So providing a complete e2e solution is usually not a requirement during the interview.
Senior software engineers have the experience of or the ability to build a complete software solution end to end. They should be experts in the domain they work in and should be able to fully own a fairly large component of a system. They should have breadth of knowledge in many aspects of the system and should have depth in a few of them as well.
System design interview is a very common way to evaluate a senior software engineer's ability to analyze and build a software system in a short amount of time. They should be able to make tradeoffs based on analysis of the requirements, come up with a complete solution using various technologies and dive into details of a few aspects of the solution when asked, e.g. high availability and scalability.
During the interview the candidate should be able to lead the conversation while focusing on the core requirements of the question asked.
Other than analyzing and designing a highly available/scalable system, a staff level engineer should also consider many other aspects of the end to end software development life cycle, such as:
You should know almost everything LOL