What are some best practices for building a scalable full stack web application and how can you ensure that your application is maintainable and easy to modify in the future

Question

As a Full Stack Developer, you know that building a scalable web application requires a lot of planning and consideration. What are some best practices you follow when building a full stack web application that can handle a large amount of traffic and user data, and how do you ensure that your application remains maintainable and easy to modify in the future?