How do I install Audacious visualization plugin?

https://pclinuxos.pkgs.org/rolling/pclinuxos-x86_64/audacious-plugin-visualizations-4.1-2pclos2021.x86_64.rpm.html is the plugin. But it is about how to install in Ubuntu which to me is quite difficult, please help!

For Ubuntu 21.04 and newer you can use audacious-plugins package from universe pocket. Install it with

sudo apt-add-repository universe
sudo apt-get update
sudo apt-get install audacious-plugins

For Ubuntu 20.04 LTS and older you have to add special PPA first, so install using commands below:

sudo add-apt-repository ppa:ubuntuhandbook1/apps
sudo apt-get update
sudo apt-get install audacious-plugins

After installation you should launch Audacious and visit Services → Plugins menu to enable needed plugins in Visualization section, drag them to needed locations to get something like shown below:

Technical details (if needed): the link in your question contains a link to rpm-package.
When downloaded and extracted, one can see its contents as follows:

usr/
└── lib64
    └── audacious
        └── Visualization
            ├── blur_scope-qt.so
            ├── blur_scope.so
            ├── cairo-spectrum.so
            ├── gl-spectrum-qt.so
            ├── qt-spectrum.so
            └── vumeter-qt.so

so the next step is to use package contents search on https://packages.ubuntu.com for example for vumeter-qt.so library. This clearly says that one needs to install audacious-plugins package.