Where to get Mojave and Catalina installers?

Your link connects to the Apple Support Downloads page, this only has updates for various OS's.

These updates do not install full working OS's.

The original question asked about accessing the App Store for the full installer apps for macOS Mojave

and macOS Catalina.

The OP has to open their Safari browser and click on the links provided by others in this thread.

As pervious posting mentioned use Safari as Other may not work.

Is certainly works using Safari in Catalina and High Sierra and yes opens a redirect and open Apple Apps Store to chosen version in How to get old versions of macOS  - just did it.

Thanks everyone! I now have what I need & used to update 2 MBPs. I had no idea the links 7 downloads only work properly in Safari! Weird!

Not really weird. The other browsers have not implemented the App Store protocol for whatever reason. I don't know if it is a technical issue or something else. I seem to think Firefox used to support it, but I might be misremembering.

Try here > How to get old versions of macOS - Apple Support

Try this: https://support.apple.com/downloads/macos

The linked page you came from works for me. Both the Mojave and Catalina links on the page open the App Store to the respective OS choices where I can then download them.

You need to open the links in Safari. Copy them then paste them into Safari's URL field.

I had forgotten about that. Good thing dialabrain didn't.

And yes, it does seem very weird the links only work correctly in Safari.

FWIW, both Brave and Chrome work, possibly other chromium browsers. Personally I just tell everyone to use Safari.