How to change Taxonomy admin page title?
Could someone tell me where to go and which static route to change?


It's not a static route.  Like other entity collection routes, it is built with the aid of the comment annotations above the Vocabulary entity class.  VocabularyRouteProvider also comes into play, and it sets a _title_callback into TaxonomyController::vocabularyTitle for good measure.

At any rate, this is all a bit moot because you don't want to go modifying core files, because then when you update core you will have to patch them all over again.  Very tedious.  Instead you could implement your own RouteSubscriberBase::alterRoutes.