macOS

Apple links to downloadable installers here.

10.15 Catalina

Newest version 10.15.3

Catalina introduced a split volume system^[1]. macOS resides on a read only volume. Data is stored on the name of the volume plus "-data". APFS is mandatory to boot Catalina. The volumes appear as one seamless drive do to hard linking and Volume Groups.

10.14 Mojave

Newest version 10.14.6

Mojave can be cloned or imaged to a jHFS+ volume. It will boot. It will not update. In order to update you will need to follow these steps

Boot to Recovery Mode.
Open Disk Utility.
Unmount the target volume.
Click "Edit" then "Convert To APFS..." and confirm the dialog box.
After the conversion is complete (it takes only a few minutes), quit Disk Utility.
Install Mojave.