Yeah, that’s why I created this post, asking for someone who can help me develop that :)

Yes, and that makes sense. Because up to now OneSignal does not know anything about your members, respective their devices a notification has to be pushed onto.
I'd recommend to find a developer to build a component which would need to connect to OneSignal's API whenever Ossn is creating a notification.

sooo, it is working now, but it's still not working with OSSNNotifications

When I send a manual notification via OneSignal Dashboard it works but it's not working with page associated notifications like messages, new likes or new comments

these are 2 different script I provided - should I just place them inside?

Like that?

</script> 
<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-2335770014631110"
crossorigin="anonymous"></script>
<script>
<script src="https://cdn.onesignal.com/sdks/OneSignalSDK.js" async=""></script>
<script>
 window.OneSignal = window.OneSignal || [];
 OneSignal.push(function() {
 OneSignal.init({
 appId: "1bacce17-0860-4b41-98b3-7e0e4dff4d74",
 });
 });
<script>

I guess I’m too stupid to understand

Could you please modify my provided code for me?

Yep, and it's indeed not what I wrote

</script>  // close the open <script> tag provided by the component

// add the 3rd party script as is here ......

<script>  // reopen the component's script part again

I added the JS file provided by OneSignal to my public_html directory and added the JS Code to the js component in the frontend like this but it is still not working. I'm not even getting the popup which tells me to accept the notifications.

Did I do something wrong?

</script>
<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-2335770014631110"
crossorigin="anonymous"></script>
<script>
</script>
<script src="https://cdn.onesignal.com/sdks/OneSignalSDK.js" async=""></script>
<script>
 window.OneSignal = window.OneSignal || [];
 OneSignal.push(function() {
 OneSignal.init({
 appId: "1bacce17-0860-4b41-98b3-7e0e4dff4d74",
 });
 });
</script>

It can be accomplished the same way I already explained 2 days ago here:
https://www.opensource-socialnetwork.org/discussion/view/6317/add-google-adsense-code