Cross-Platform Realtime App

Hi, I'm going in loops between cloud providers trying to find out how to integrate DB + API in realtime.
What is what you guys use, what tools, websites, hammers, something... to create let's say a realtime todo app that works in macOS and iOS?

I tried Firebase and it's not compatible with macOS, the same thing with AWS Amplify.

What are the right tools to achieve this? Please help

Thanks,
Arturo