I just added pwa functionality to an older project at work. Although I haven't tested thoroughly and although my project wasn't an Angular project, this has worked so far:

• Add a service worker. You'll need one that defines a fetch event listener. The one I use I found online. It looks like this:

// sw.js
//Install stage sets up the offline page in the cache and opens a new cache
self.addEventListener('install', function (event) {
    var offlinePage = new Request('offline.html');
    event.waitUntil(
    fetch(offlinePage).then(function (response) {
        return caches.open('mesquite').then(function (cache) {
            console.log('[PWA Builder] Cached offline page during Install' + response.url);
            return cache.put(offlinePage, response);
        });
    }));
});

//If any fetch fails, it will show the offline page.
//Maybe this should be limited to HTML documents?
self.addEventListener('fetch', function (event) {
    event.respondWith(
      fetch(event.request).catch(function (error) {
          console.error('[PWA Builder] Network request Failed. Serving offline page ' + error);
          return caches.open('mesquite').then(function (cache) {
              return cache.match('offline.html');
          });
      }));
});

• Register the service worker:

// this javascript code runs on the the layout markup of a MVC NET app. 
if ("serviceWorker" in navigator) {
            if (navigator.serviceWorker.controller) {
                console.log("[PWA Builder] active service worker found, no need to register");
            } else {
                // Register the service worker
                navigator.serviceWorker
                  .register("/sw.js", {
                      scope: "./"
                  })
                  .then(function (reg) {
                      console.log("[PWA Builder] Service worker has been registered for scope: " + reg.scope);
                  });
            }
        }

• Add a manifest

{
  "name": "ProjectName",
  "short_name": "ProjectShortName",
  "description": "Description",
  "theme_color": "#00BCD4",
  "background_color": "#fafafa",
  "display": "standalone",
  "start_url": "/",
  "scope": "/",
  "prefer_related_applications": false,
  "icons": [
    {
      "src": "Images/Logos/logo-sm.png",
      "sizes": "192x192",
      "type": "image/png"
    },
    {
      "src": "Images/Logos/logo-lg.png",
      "sizes": "512x512",
      "type": "image/png"
    }
  ]
}

• Reference manifest.json

  // layout.html
  <link rel="manifest" href="/manifest.json">
  

• Instead of prompting the user to install the app, I show a download button so that the user can download it if they want to/when they want to. That logic looks like this:

 const a2hsButton = document.querySelector('#a2hs');
    if (a2hsButton) {
        let deferredPrompt;
        //a2hsButton.style.display = 'none';
        window.addEventListener('beforeinstallprompt', function (e) {
            console.log("beforeinstall");
            // Prevent Chrome 67 and earlier from automatically showing the prompt
            e.preventDefault();
            // Stash the event so it can be triggered later.
            deferredPrompt = e;
            // Update UI to notify the user they can add to home screen
            a2hsButton.style.display = 'block';

            a2hsButton.addEventListener('click', function (e) {
                // hide our user interface that shows our A2HS button
                // Show the prompt
                deferredPrompt.prompt();
                // Wait for the user to respond to the prompt
                deferredPrompt.userChoice.then((choiceResult) => {
                    if (choiceResult.outcome === 'accepted') {
                        a2hsButton.style.display = 'none';
                        console.log('User accepted the A2HS prompt');
                    } else {
                        console.log('User dismissed the A2HS prompt');
                    }
                    deferredPrompt = null;
                });
            });
        })

Hopefully it helps some. Checking out PWABuilder will help too.