I've been using MDS to download macOS installers but that is one feature of a rather big enterprise admin type utility. MDS 5.0 requires a license which is overkill. But I was looking at the 4.3 release notes and it noted that it replaced the installer downloader code with Mist.

Mist is "A Mac utility that automatically downloads macOS Firmwares / Installers." Launch Mist and it shows you a list of available macOS install downloads from Apple (and it generates the list from the Apple download site). For each entry you can click to download the installer or download and create a bootable installer. If you pick an installer incompatible with your Mac CPU then it'll warn you.

For download installer you can choose to have it save the app or the app in a disk image/ISO/package. It then shows an informative progress checklist so you can see where it is in the process (when downloading it shows download size and how much downloaded). Mist will download the installer, verify it, and unpack/repackage it as needed. (BTW downloading from Apple's site is quite fast, about 450 to 500 Mbps with Fiber Internet + 802.11ac WiFi).

For creating a bootable installer it needs a disk (or partition) that it will erase then create the bootable installer on.

Mist can also download firmwares but not sure that those are.

Anyways, an easy way to download installers which you then use to install macOS on a machine or a VM. Useful if you want to install not the latest macOS since Mac App Store won't really allow you to install older OSes. Also useful to install on a VMWare Fusion VM since Fusion wants an install image to work with (it won't download it for you). Theoretically you can also use the Mac App Store to download the installer as if you're going to upgrade then cancel and get the installer from the download location but that's a bit tricky.