Why are some shortcuts modified with the Shift key while others are modified with the Option key?
Well, coming from a Windows background myself, I can state the following:


The Cmd key is usually like Ctrl on Windows (e.g., Ctrl+c == Cmd+c).
As with Windows, the Shift key is a common modifier to the Cmd key.
I believe that Macs traditionally had an Option key, which also came to be known as the Alt key (perhaps to help those accustomed to Windows).
Aside from that, both Ctrl and Option are treated like modifiers on the Cmd key.


Between different Mac applications, there is sorta an effort to have some consistency in what certain key combinations do. For example, Ctrl+Cmd+f is usually a full-screen toggle.

However, the consistency of such keyboard shortcuts can vary per the software publisher.

Now, as a person that is familiar with Windows, you'll notice that Alt doesn't activate a menu mode, such as being able to drill down to a specific menu item.

However,you can create a keyboard shortcut via System Preferences >> Keyboard >> Shortcuts >> App Shortcuts. Just type the shortcut name as it appears in the application's menu.