Btw as this thread is dead anyway as the OP has ragequit in more detail as far as I understand it works like this:
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When you load the link in the post it activates the related function in the forum software (discourse) and looks up the page the link is pointing to (this happens client side I guess so it is your computer that looks up the site).
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Steam store provides the gameâs store page dynamically based on the requestâs geolocation (yours, the server provided as your source location, VPN might alter it which is the very purpose of such but depending on your provider the geolocation might be incorrect or even deliberately obfuscated) thus if someone through a German source looks it up it provides the German page by default (or at least the local currency associated with Germany) and so on.
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The forum software pulls the data from the loaded steam store page and creates the preview then inserts it in the post. (Thus you will see the details or at least the currency according to your local one, or the one the geolocation points out as the source, though it might be imprecise.)
All this happens in almost an instant though. At least this is my educated assumption how this works, which might be not entirely accurate in the end though.
Edit: Note that discourse is like a complex script, not just a basic static or simply scripted forum like the old ones, it is a full program in script form that is running in your browser.