Essentially, you scan a barcode which means notifications are received via Matrix but only so long as your device is on.

If you have a physical server, you might want to opt for a cheap device that can sit next to your server, essentially meaning you're online 24/7

If you use a VPS, notifications will still be forwarded but only work while your phone is powered on. It's not a true replacement for Whatsapp as you still need it running on a device somewhere.