The TL;DR is that being a Senior Engineer means being you are helping steer the ship. It does not just mean that you know more stacks and frameworks. Your job as engineer changes over time. Starting off (New grad, E3, etc.), you will be assessed almost entirely on your execution. If you consistently deliver high quality code on time and start to take some ownership over your work — you will be successful and grow. Next (Mid-level, E4, etc.), you are expected to exercise ownership and direction over a well-defined area. Your execution and coding skills should be at a solid bar. What will lead to your growth at this level is showing your ability to take on and solve ambiguous problems, without much guidance. This usually involves leading others. At this point you will be considered "Senior". Notice how your direction and people skills become more important over time relative to your engineering skills. Your job slightly changes at each level. As a senior engineer, you are expected to have great engineering skills but your impact also comes from influencing decisions, making sure the team is moving in the right direction, and being the one to inject context which drives consensus in an organization. A short answer in a comment cannot do justice to the question, if you are interested in learning more I highly recommend the book "Staff Engineer" by Will Larson. This will give you a very clear understanding on what an engineering career looks like even past "Senior" and what skills you will need to develop to get there.

This is incredibly useful! It seems like being able to find solutions and leading others are two of the many skills I should be looking to advance. Thank you for the book recommendation, I will have to check that out!