Download YouTube Transcript - open the Transcript panel on a video then execute this function in the developer tools console

download-transcript.js

function downloadTranscript() {
  let $segments = document.querySelector('.ytd-transcript-search-panel-renderer #segments-container')
  let sections = []
  let parts = []

  for (let $el of $segments.children) {
    if ($el.tagName == 'YTD-TRANSCRIPT-SECTION-HEADER-RENDERER') {
      if (parts.length > 0) {
        sections.push(parts.join(' '))
        parts = []
      }
      sections.push(/** @type {HTMLElement} */ ($el).innerText.trim())
    } else {
      parts.push(/** @type {HTMLElement} */ ($el.querySelector('.segment-text')).innerText.trim())
    }
  }
  if (parts.length > 0) {
    sections.push(parts.join(' '))
  }

  let $link = document.createElement('a')
  let url = URL.createObjectURL(new Blob([sections.join('\n\n')], {type: "text/plain"}))
  let title = /** @type {HTMLElement} */ (document.querySelector('#above-the-fold #title'))?.innerText ?? 'transcript'
  $link.setAttribute('href', url)
  $link.setAttribute('download', `${title}.txt`)
  $link.click()
  URL.revokeObjectURL(url)
}