Davey Tree's customer portal serves a couple hundred thousand people, and I still work on it. The interesting part is that the data lives in two systems at once, SAP and Dynamics 365, and the portal has to present them as one.
It's a Vite React app now, but it's old enough to have started life on Create React App. My recent work has been modernization: the backend from .NET 6 to .NET 10, Node from 18 to 24, and a migration of one data source off SAP and onto Dynamics. The piece I liked most was tearing out the SAP CDC UI forms the app leaned on and replacing them with custom React forms that talk to the CDC API directly.