Communicate from Parent to Child

The @api decorator in the child component exposes a property, making it public, so that the parent component can update it.

Similarly, @api decorator function in child component can expose it as public to call it from the parent component.

Check this for sample code and implementation

How to pass data from parent to child component using LWC in Salesforce?

Communicate from child to parent

CustomEvent can be used.

Check this for sample code and implementation

How to pass data from child to parent component using LWC in Salesforce?

Communicate between Unrelated Components

Lightning Message Channel can be used.

Check this for sample code and implementation

Lightning Message Channel in Salesforce