The same way they find out that they have to press S to start it – the manual, or the tutorial videos. Slurs behave like tuplets and other objects that can be ‘nested’.
I’m not convinced that you can obtain all the information needed to use any app just by looking at the icons and menus.
Some things toggle, but not most things. Ties, gracenotes, tuplets, fermatas, playing techniques, clefs…
I’m guessing you’re on Windows? It seems (from what I can gather in posts here) that Windows audio drivers are variable. Doesn’t the Preference for default mixer level fix this?