"Watch folders" function doesn't work

You don't state which Elements version ?

When adding new files to a folder, nothing happens!

You should not expect the watched folder function to work immediately. I have not found any tech doc about it, but in my tests, it seems that in PSE2019, the search only begins after a restart of the organizer.

Where to find manual start of scanning folders for changes?

There has never been a function for manual scan for watched folders.

That's the reason why I don't consider it is useful at all for me.

After all, without watched folders, you simply import from the folder. Files already present in the catalog are skipped and new ones are imported. I don't like automatic background processes which take too much resources when you are editing or organizing. I prefer to choose when to start the import myself, very often when I am away from the computer. Since you can check the last import batch, it's even easy to cull photos you don't want to keep.