When I started as a junior engineer, my work day mostly included fixing bugs and helping build new features for our web applications. I would come into work in the morning and check my email/bug tracker for tasks that were assigned to me by my manager and spend the day investing bugs, fixing them and verifying that the fixes worked. I would also have regularly scheduled meetings with my teammates to discuss priorities of what we'd be focusing on, such as which bugs were high-priority. I usually had a mentor engineer that would review my work, be available to answer my questions and give me guidance on how to build software.
When I eventually grew into a mid-level engineer, my responsibilities included building new features for our web applications. I was also working without much mentorship, I was expected to complete my tasks on my own. I was responsible for working with a project manager, designer and back-end engineers to plan and build out new functionality for our project. The project manager knows all about the new feature we're building and they're the person we ask product-related questions to (how do we want users to interact with our app?). The back-end engineers would handle things like processing the credit card payments, sending out emails, creating databases to store user data, etc. The designer creates visual prototypes of what our users will see. As the front-end developer, I build the web pages that would show things like the sign-up form and credit card fields and anything else that the users would see on their computer screen. Building these new features as a team could take anywhere from days to months.
As a senior engineer, I can build entirely new web applications (instead of just adding new features to an existing app) and assist other junior and mid-level engineers with their technical growth. The architecture decisions made at the start of a new project are very important, because they can stick around for years and have a big impact on how fast we can add new features (good architecture will make it quick and easy). To help other engineers with their technical growth, I learn about their interests and goals and work with them to make sure they get the types of tasks and projects they will enjoy and grow from.