Assuming you’re using the SAML module, you just need to set the DefaultLogoutPage constant to the page/url where you want users to end up after logging out. The documentation here describes the setting in more detail.
Functionally, this works by setting a cookie on your user’s browser called “originURI”. When a user logs out, they are sent to the location specified in that cookie. You’ll see code referencing originURI in either the default index.html or login.html files.