Just a push notification

I’m slowly going crazy here.
I’m looking for a way to send push notifications to app users.
But somehow I just can’t get any further here. OneSignal doesn’t seem to really work anymore without appropriate extensions.
I find a free extension here (https://community.kodular.io/t/free-onesignalpush-integrate-latest-onesignal-sdk-5-0-2-in-your-app-to-send-push-notifications /236720) but I’m apparently too stupid to use this.
After my experience, I didn’t want to buy paid versions.

Maybe one of you can give me an idea of which extension you can use or even say that it is possible again to take the OneSignal extension directly from Kodular.io.

Or something to finally make it work.

Let others help you by showing what makes you think that.

I have now tried to install the free extension from OneSignal.
Unfortunately I couldn’t find any instructions or anything like that.
So I started tinkering around a bit. But simply registering the device with OneSignal is not possible for me.
What I mean is that I don’t even really know how to give users a token at OneSignal.

I hope I made myself understandable somehow.
You first have to deposit the appid and register the device with OneSignal.
But even for that I somehow lack the understanding