Generally speaking, the key that serves a role similar to the Alt key on Windows is the Option (⌥) key. It's located right next to the Command (⌘) key, which replaces Ctrl on the Windows keyboard.

The Command key - aka Apple key or Cmd key - gets a lot of use. Here's where it is on a Mac and PC keyboard, and what you can do with it.

Press and hold the Option key as soon as your machine restarts. Release when the Startup Manager window opens (or when you’re asked to put in your firmware password). ... On a Windows PC.