Discord is distributed as a .Deb if you don't use flatpak because they can't be bothered to set up a repo.

The very useful thing about local file install is that unlike dpkg, apt will install dependencies automatically

apt and apt-get both use dpkg internally, but these days it's essentially seen as an implementation detail that regular users don't need to know about.

dpkg doesn't resolve dependencies (that's a feature of apt) which means that if you install a Debian package with dpkg, you'll have to manually install all dependencies first, and they won't be marked as automatically installed (so autoremove won't remove them if they're not needed any more). Using apt solves that.

The web suggests dpkg because either the articles are old, or they're based on outdated knowledge :)