How to implement push notifications on the site?

Question

cloudflare

Such a task: it is necessary for the user to receive push notifications with content that will fit into the admin panel.
1) Can notifications come if the user does not have my site/browser open?
2) Is it possible to implement using setInterval and ajax, or is it nonsense and will heavily load the user's browser?
I read about google firebase, but somehow I was not inspired.

Answer 1

I found free SSL, https://www.sslforfree.com/ and then, using js knowledge, I made push notifications + https://push.world/

Answer 2

https://onesignal.com/