Hi @Lăng Vi ,
I did run into the same issue you did when I created a reactjs app using create-react-app npm package. When I deployed the application to an Windows OS, the page came up as expected. Checking the log files, revealed .NET Core is running /defaulthome/hostingstart/hostingstart.dll, which makes sense because we're running a .NET Core on a Linux host and why we're getting that default page. To get around this, you can wrap your react app in a
dotnet new react -o myapp as instructed in https://learn.microsoft.com/en-us/aspnet/core/client-side/spa/react?view=aspnetcore-3.1&tabs=visual-studio. But since .NET Core 3.1 hasn't been deployed to all regions, you're safer switching to .NET Core 3.0. If there's a new feature in 3.1 that your app requires, they you'll need to switch to a docker image.
Also, bear in mind that FTP'ing your content to your web app doesn't execute package restoration. So you'll need make sure you create your necessary dependencies and config files before-hand and FTP along your with artifacts. See https://learn.microsoft.com/en-us/azure/app-service/deploy-ftp#deploy-files-to-azure for more information.
Hope this helps.