Thanks for your hint about the feature request assign-tag-to-multiple-files.
I read this topic and found the Youtube Video where Nicole van der Hoeven showed the “search and replace” feature and I think, even it is not automated, this could solve my problem too.
First I add all (urgh 
) articles in my vault. Then, when I read a new one and there is place A, I search all old articles for this place and replaced the place in the text with a tag (or link, not sure, what is better).
Sure, if obsidian would automaticly see the existing links/tags in the new artice it would be awesome, but I need to read the new one anyways, so I could add the tags (or links, not sure, what is better 
) on the fly and would do a search and replace action with all old articles too and would replace the place A in the text with a tag (or link).
But the automated feature would be nice :D.
That means: I need a search and replace tool in obsidian which crawl through all notes in my vault. Not just a search and replace possibility inside a note.