@Tomexx that looks ok… but your words above are confusing…

What I couldn’t figure out is how to install MMM-WeatherForecast or MMM-Wallpaper.

After installing both of them, they never appear, not even trying to load.

they ARE in the modules folder, yes?

standard rules, ALL modules get installed in the modules folder
generally they come from github so the download process is

cd MagicMirror/modules git clone ?????

where ??? is the url provided by the github repo for the module.
you find the module repo (google, or whatever), then push ‘clone or download’, and push the button (looks like clipboard with and arrow pointing to it) to get the proper clone url string, then paste that string after the words git clone, and hit enter

then follow the instrucitons for the module to complete its installation and config

some modules need you to run npm install to get other code they depend on installed. neither of these do