Why does macOS kept auto correcting z to s?

Solution 1:

I'm not sure where in the settings I changed accidentally but now wherever I type, in browser, note, or emails etc, my s always gets auto corrected to a z.

Assuming you are on a Mac, go to System Preferences > Keyboard > Text > Spelling: and either select the language you are using or set it to the default, Automatic by Language.

Note that you may also want to check what you have set, or if you changed from the default settings, from the Set Up… pop-up menu item after clicking Automatic by Language pop-up. If you have not modified anything in the Set Up… from the installed defaults you should not hav an issue with using Automatic by Language. This of course is assuming that U.S. English was your original language of choice and not British English which uses s in places where U.S. English uses z in those instances it's being auto-corrected.

On a default U.S. English install, this is what the sheet from the Set Up… pop-up menu item looks like:

Solution 2:

Apple's (and other) British English spellchecker favours -ise endings instead of -ize, for some reason**. It sounds like you've got British English selected.

**(Interestingly, -ise in British English is a recent development, and the Oxford English Dictionary still gives -ize for most words as the preferred spelling.)