The hardest answer: Don't download. Use the native offline features of apps (Netflix, Spotify, YouTube Premium). They encrypt the downloads to your device, so you can't share them, but you can watch them on a plane.
Video DownloadHelper is a popular browser extension for capturing media, it generally cannot download DRM-protected content video downloadhelper drm
appearing next to the URL indicates the content is DRM-protected and likely incompatible with the extension. Result of DRM Downloads The hardest answer: Don't download