using QEMU / KVM manual
Running macOS not on Apple hardware is quite not easy and full of surprises. A working way is the use of QEMU / KVM on any Linux you like. You will find a detailed howto inclusive needed scripts at foxlet’s github account. This is the recommend solution, if you plan to use it in a complex scenario.
Currently you can install macOS High Sierra (10.13), Mojave (10.14) or Catalina (10.15).
sosumi
A really simple and for most cases sufficient solution is a snap based application named Sosumi. It is based on the scripts of the first named solution. You can easily install it via snap:
snap install sosumi
After that you can simple start it via command:
sosumi
The snap will download all necessary installation media and create a 64 GB disk image file (QCOW2) in your home directory (~/snap/sosumi/common/macos.qcow2). So check if you have enough free space. It will then install macOS catalina.