Create custom Google Smart Home Action - actions-on-google

Question

I have a Google Nest Hub Max and I want to increase its capabilities for a custom need:
"Hey Google, add xyz to my work planning"
Then I want to make an HTTP call to my private server
The private server returns a text
The text is displayed in the Google Nest Hub Max screen + speak-out.
How can that be achieved?
Originally I thought that this will not be difficult. I've imagined a NodeJs, Java, Python or whatever framework where Google gives me the xyz text and I can do my thing and return a simple text. And obviously, Google will handle the intent matching and only call my custom code when users say the precise phrase.
I've tried to search for how to do it online, but there is a lot of documentation everywhere. This post resumes quite well the situation, but I've never found a tutorial or hello world example of such a thing.
Does anyone know how to do it?
For steps 2. and 3., I don't necessarily need to use a private server, if I can achieve what the private server does inside the Smart Home Action code, mostly some basic Python code.

Answer 1

First - you're on the right track! There are a few assumptions and terminology issues in your question that we need to clear up first, but your idea is fundamentally sound:
Google uses the term "Smart Home Actions" to describe controlling IoT/smart home devices such as lights, appliances, outlets, etc. Making something that you control through the Assistant, including Smart Speakers and Smart Hubs, means building a Conversational Action.
Most Conversational Actions need to be invoked by name. So you would start your action with something like "Talk to Work Planning" or "Ask Work Planning to add XYZ'. There are a limited, but growing, number of built in intents (BIIs) to cover other verticals - but don't count on them right now.
All Actions are public. They all share an invocation name namespace and anyone can access them. You can add Account Linking or other ways to ensure a limited audience, and there are ways to have more private alpha and beta testing, but there are issues with both. (Consider this an opportunity!)
You're correct that Google will help you with parsing the Intent and getting the parameter values (the XYZ in your example) and then handing this over to your server. However, the server must be at a publicly accessible address with an HTTPS endpoint. (Google refers to this as a webhook.)
There are a number of resources available, via Google, StackOverflow, and elsewhere:
On StackOverflow, look for the actions-on-google tag. Frequently, conversational actions are either built with dialogflow-es or, more recently, actions-builder which each have their own tags. (And don't forget that when you post your own questions to make sure you provide code, errors, screen shots, and as much other information as you can to help us help you overcome the issues.)
Google's documentation about how to design and build conversational actions.
Google also has codelabs and sample code illustrating how to build conversational actions. The codelabs include the "hello world" examples you are probably looking for.
Most sample code uses JavaScript with node.js, since Google provides a library for it. If you want to use python, you'll need the JSON format that the Assistant will send to your webhook and that it expects back in response.
There are articles and videos written about it. For example, this series of blog posts discussing designing and developing actions outlines the steps and shows the code. And this YouTube playlist takes you through the process step-by-step (and there are other videos covering other details if you want more).