Systems communicate via components. They can create empty "marker" components (and remove them again as needed) or, when applicable, put data in shared singleton data structures called "resources."

In some ECS in Rust (like Specs) there is a concept of a “resource” for this. System declare their use of resources, which allows the scheduler to order them.

I like to have a message bus to send messages to queues and have systems consuming those queues.

The shared mutable reference to this message bus would be provided to each system requiring it. It's easily achievable in Rust with Bevy, or with Unity DOTS.

When I don't use Unity DOTS and simply good old MonoBehaviours, I call the message handlers as soon as the message is published instead.