>>283681280 (OP)
>What's the deal with nosebleeding in anime and manga?
Berri - Berri / and enemia ( Vitamin B deficiency / low iron due to Japanese diet compounded with poverty in the 1940's ) led to the idea / belief that if you got too excited you would get a nose bleed.
Add to that drawn Manga -- you have a character sexually aroused by some event . You have to pass this on to the reader.
You have two choices --
The Character announces that he is aroused, or works it in to the conversation.
You draw the erect penis. ( not allowed by censors )
So instead you insert the nose bleed trope and the reader gets the reference -- every other Manga and Anime copies it to solve the same problem.