The search result is a minefield. Don’t click the first link.
Apple maintains a hidden support page for legacy installers. While Apple prefers the .app format, you can easily convert it.
Then retry the installation.
If you cannot access Apple’s servers (e.g., using Windows or a very old Mac), you might turn to third-party archives.
No. The installer is a macOS .app bundle. You would need a friend’s Mac or a virtual machine to prepare the DMG. Download Macos Sierra 10.12 Dmg File
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
A DMG (Disk Image) file is a container that holds the macOS installer. Unlike running the upgrade directly from the App Store, a DMG allows you to: The search result is a minefield
Not directly. You can: