Opel Project

Hello CAN community,

First, I'd like to briefly introduce myself and talk a little about my current project. I hope to find some interesting information and help here.
I'm from the outskirts of Munich and have been running a fairly successful online forum about cars and tuning for several years. It's a topic that's quite rare in the Munich area. I drive an Opel Vectra C GTS V6 with all sorts of extras, including a car PC, etc. Eventually, I'll want to display more vehicle data than just the currently playing song. But until now, I haven't been motivated to delve into the topic of 'how to get data from the CAN bus.' However, one of our forum members recently bought an Opel Astra H OPC, and now the topic is becoming very interesting. This car doesn't even have a coolant temperature display. There's no indication, let alone any information, about the turbocharger pressure. Now I know that these values are all available in the car, on the CAN bus, but the question is - how do I access them?
So, completely unaware, I bought an ELM-CAN adapter with all the necessary accessories from an online retailer (which I'm sure most people are familiar with). What can I say? It works – I can read and visualize the corresponding standard data. Okay - but what happens next?
From certain sources, I know that when I'm connected to the CAN bus, I should be able to read all the data – but only if I know how. For example, I already know the PID number for the boost pressure. But how do I retrieve the value? Is it even possible to achieve this goal with the hardware I currently own?
And just to be explicit, I don't want to write or send anything to the bus. I just want to read out a few values so that I can ultimately display them in the CID (center information display) of the Astra using a car PC.
Perhaps someone here can help me with some basic questions.

Best regards from Munich.