I haven‘t yet found an official Apple website that lists older macOS versions. Fortunately, I have kept the installers on a separate external storage after each major upgrade, but when launching them from a macOS version newer that the installer itself, macOS shows an error that it‘s not possible to install it. What is the official way of installing older macOS versions so that I can test my App Store apps?
You can use your installers to create bootable usb installers (using the "createinstallmedia" command).
A quick trawl of the Internet will show various guides.
It may not be wise to downgrade over a newer version... I suspect erase-and-install would be the safest option.
If your problem is more complex than your original question, you will need to add more information.
e.g.
What are your "installers"?
creating an installer on an external device doesn't work
"Doesn't work" how?
I mentioned bootable usb installers... that's not just any "external" installer.
Per Apple: How to get old versions of macOS. This covers versions from the last ~seven years.
As for testing the older installers, you'll have to test them without newer software around, and/or test them using a bootable drive and createinstallmedia.
The Mac involved can need to support the macOS or OS X version you are testing with, too.
The usual workaround for a busted certificate in an old installer is to manually set the system time back to when the version just was released, launch the installer and stop at the first prompt, switch back to system preferences and re-set the time to current.
I asked:
What is the Mac?
You didn't reply...
...but if you have an M1 Mac, then that won't support Catalina or older.
Otherwise, have you considered using Parallels, to run older versions of macOS as virtual machines?