How to implement multitasking in ruby?

Question

ruby

Hi all.
The idea was born here to develop a small application in pure ruby.
What is needed:

1. development only in pure ruby ​​(without frameworks, etc.);
   
2. creating a separate process or thread to perform some task;
   
3. the client can run background processes (threads) on the server that can send notifications to the client;
   

There are still a few sub-questions here:
What is better to use processes or threads?
Starting processes (threads) should be performed by the server or application?
Thank you for your attention!