Writing jsw code and it doesn't work well - need help

I’m writing some backend code that the frontend will use. Once I open the file under src/backend - let’s say it is called “mybackedn.jsw” , the tsconfig is alerting as red and I can’t use the IDE and completion any more. The code can run, but without completion which creates a lot of hurdle. Anyone has an idea what am I doing wrong?

While .jsw might still work, it is being phased out in favour of using webMethod()

Might be related, but here are the docs for using webMethod - Web Method | VELO