On page seven vs. on [a/the] page seven [closed]

The implied name of the location is 'page seven'; that carries with it the meaning that the place is a page. Naming it 'seven' would be too vague as there might also be a chapter called 'seven' and a volume called 'seven'. So "it's on page seven" is fine as the name is all you need. The alternative would be "it's on the seventh page," - also fine, because you are then identifying the page by position, not by name.

This pattern is repeated elsewhere. You wouldn't say "take this suitcase to the room seven," just as you wouldn't say "the suitcase belongs to the Konstantin." However, you might say "take this suitcase to the front bedroom." In that case, 'the front bedroom' is not a name, only an identifying description (with preceding adjective).