What is the "Ctr" key? Do you mean "Control"? Harder question - what is the "Alt" key? Mac keyboard don't have one. There is an "fn" key, a "control ^" key, an "option ⌥" key, and a "command ⌘" key. While this forum happens to map Command + Control + Space bar to the Character Viewer, that is not a system-wide behaviour.
System defaults could be a couple of different things depending on what the actual keystrokes you are using. "Control + Option + Space" is defined in System Preferences > Keyboard > Shortcuts > Input Sources as "Select next source in input menu". "Option + Command + space" is defined in Spotlight as "Show Finder search window". Of course, you can change all of those. You might even have an "Alt" key if you are using a PC or 3rd party keyboard, but it should be mapped to "Option".