Does that still work when your internet is turned off? Some. People are lucky that they always have reliable internet, others do not.
Pushover
Pushbullet
Telegram
My openhab cloud
Plenty of ways but I believe they all will require a cloud and internet to work.
Self host it on a separate machine or even the same machine that openHAB runs on.
On Android it uses WebSocket communication for real time notifications.
Here’s a sample rule to send a notification when a sensor is triggered:
rule "Hallway Motion"
when
Item motion_hallway changed
then
if (motion_hallway.state == ON) {
sendHttpPostRequest("https://<ntfy-url>/<ntfy-topic>?auth=<auth>", "text/html", "Hallway Motion")
}
end