How Can I Send Notifications from a Web App to My Phone?

These are known as web push notifications. Like others said, you'll definitely need a server to send these notifications, as well as a database to keep track of the push subscription URL. Also, make sure your users perform some extra steps like adding the web app to their home screen for the notifications to work properly.

Check out Web Push Notifications! You can find a lot of helpful information on MDN. You'll essentially need to implement a server-side setup and use a service worker on the client side. Here's a simple JavaScript snippet to get you started:
```JavaScript
async function subscribePush() {
const registration = await navigator.serviceWorker.register('your_service_worker_implementation.js');
const subscription = await registration.pushManager.subscribe({
userVisibleOnly: true,
applicationServerKey: 'your-public-vapid-key'
});
// Send subscription to your server
fetch('/subscribe', {
method: 'POST',
body: JSON.stringify(subscription),
headers: { 'Content-Type': 'application/json' }
});
}
```
Just remember to set up a way to actually send those notifications out from your server!

You might want to look into Progressive Web App (PWA) notifications. It's a way to send notifications directly to devices from the web, but you'll need a server to handle the process since you can't send messages peer-to-peer.

If your app is a web app or even a native one, check out wappaa.com. It might help you integrate notifications more easily!