According to Google maps: there’s no definite border (indicated by dashed lines) between the Kashmir region (located near both India & Pakistan) in which India considers the entire region as theirs but both China & Pakistan argue against India’s claim.
Short answer: British
Long answer: When India got independence from Britain, each province and princely state was asked to choose whether to join India (secular, Hindu-majority) or Pakistan (Muslim majority, with legal protections for minorities). Kashmir’s Hindu king decided to join India, even though its population was majority Muslim. Three wars later, most of Kashmir is now in India, while the westernmost part is under Pakistan. Politicians in both countries have sabre-rattled and done other stupid shit to inflame the situation.
The India-China dispute is because the British Empire promised that bit of land to both India and China. Now it’s controlled by China, as part of Tibet Autonomous Region.