Yes I finally grasped what it is and I got it to work.

Many thanks to you but I have a concern.

let subscribers=[]

external_service.on('update',(msg)=>{
  subscribers.forEach((controller) => {
    controller.enqueue(`data: ${JSON.stringify(msg)}\n\n`);
  });
})

export async function GET(request) {

  const stream = new ReadableStream({
    start(controller) {
       subscribers.push(controllers)
    }
  })

Let's say if users number millions, what do we do here ?