This is an excellent post that takes you along on the author’s journey to build a simple, collaborative (desktop-like interactions and realtime collaborations, such as Notion, Discord, Figma, etc.) todo app:

With the help of many great tools, we’ve successfully built a fast, collaborative todo app. More importantly, we’ve worked out a reasonably simple approach to building similar web apps. As the user base and feature set grow, this approach shall scale well in both performance and complexity.

Both the resulting frontend and backend code are open source and you’ll likely learn about some new tools (like Replicache) along the way.