JSFiddle - React, Tailwind, and code Playground

JavaScript

const arr = [
  ["Mallory", "Everest", "Mont Blanc", "Pillar Rock"],
  ["Mawson", "South Pole", "New Hebrides"],
  ["Hillary", "Everest", "South Pole"]
];

const result = arr.reduce((result, items) => {
  const [explorer, ...toponims] = items
  toponims.forEach(item => {
    if (result[item]) {
      result[item].push(explorer)
    } else {
      result[item] = [explorer]
    }
  })
  return result
}, {})

const resultAsArray = Object.keys(result).map(item => [item, ...result[item]])

console.log(result, resultAsArray)