I've just switched to macOS. I'm wondering if there is a way to show Launchpad and Spotlight using single button shortcut, like Windows does with Window key and the Start bar?
It would be very handy to me (possibly without external softwares..).
MacBook keyboard and Apple Magic keyboard comes with dedicated key to invoke Launchpad. The special functions are labelled on the row of function keys (and equivalent in case of MacBook Pro equipped with Touch Bar). Look for key labelled F4 on your keyboard.
If you are using a non-Apple keyboard, go to System Preferences → Keyboard → Shortcuts → Launchpad & Dock and select the checkbox next to Show Launchpad. You can also double click on the keyboard shortcut and change it to something else.
You can also define custom keyboard shortcut(s) easily by going to System Preferences → Keyboard → Shortcuts. Since you are essentially looking for single key to invoke the desired operation(s), you can do so by creating a single key shortcut (if not natively provided by macOS).