This question already has an answer here:
How to make Illustrator default to CMYK. My color box says CMYK, but my swatches still default to RGB please help.
0
marked as duplicate by Luciano, Paolo Gibellini, Billy Kerr, joojaa Mar 24 '18 at 18:28
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
2
Illustrator "defaults" to what an existing file is set to, or whatever you tell illustrator to use. There's no way to tell Illustrator to always use a particular color mode.
Open new files as CMYK:
OR
Illustrator uses whatever color mode you chose when opening a new file.
If you are opening existing files, you'll need to change the document color mode via File > Document Color Mode: SEE HERE
You should also be aware of issues with merely switching document color modes: See here
