Do you know any notification systems that react to given events?

Question

Notifications

Good day, gentlemen! I turn to knowledgeable people because of my extreme ignorance. I am interested (even rather not me, but the manager) notification systems (alerts, notifications). This mythical system should be able to process the specified events using notifications (sms | email). The system should be a "separate module", i.e., as I understand it, either some kind of ready-made software (having an API), or, for example, some kind of library, although the former is more likely.
Apparently, the manager intends to somehow connect this with the company's server, but he gave me an example with a user registration notification on some site, which puzzled me even more. In principle, any such systems, including systems for the web, will work for me, I’ll figure it out somehow. I understand that monitoring systems like Nagios, Zabbix, etc. do not belong here. Sorry for the clumsy wording, that's all there is.
Ideally, I should make a list of such systems, but if you name at least one such system, then I can build on it and, I think, I will find others.
Thanks in advance to all who respond!

Answer 1

Events are happening on your side. And you can already send it through the API, for example, through PushAll, you can send a notification immediately to smartphones, computers, tablets, etc. moreover, in telegrams you can use push notifications and email as well.

Answer 2

in general, Nagios, Zabbix just relate,
but the manager wants something like
https://ifttt.com/
or in general an advanced CRM
with direct hands, you can notify anyone / anything about anyone about anything, if you're lucky and google - you will find what -something ready

Answer 3

https://en.wikipedia.org/wiki/SIEM