Domoticz

I'm completely new to the mqtt stuff.
I looked everywhere and couldn't find something like a step by step howto how to assign a dimmmer 2 to domoticz.
I tried with node-red as well unfortunately my understanding is not sufficient.

Well.... it is not really complicated

First off, there is an API documentation from shelly where you can find everything for MQTT that is specific for your device:
https://shelly-api-docs.shelly.cloud/ge ... 2-overview
(assuming that is your device)

Secondly, once you get into Node-Red it will really shift the limits of your integration capabilities.

I would first create a flow in NR, that integrates your hardware device . that is pretty simple and does not require a lot. Also there's plenty of example flows out there. And you don't need to integrate all options, but only what you need, especially when it comes to the data from your dimmer.
on the input side there is only 3 options anyway: on, off, brightness.

Once you have that, you can integrate that into domoticz, which is even more straightforward.

I'm using around 20 Shellies (but no dimmer) via Node-Red, so I may be able to help out a bit with the details if required.

A Step-by-step instruction will get you to a result, but you don't understand why then. And where would be the fun in that...?