I’d use a Search/Replace plugin (I like “Better Search and Replace”) to change all instances of ‘olddomain’ to ‘newdomain’.
Then I would look at the wp-options table to ensure the new URL is in there (in two places). I’d remove any URL-based settings in the wp0config.php file. (I never change those.)
I’d also check the htaccess file in the new site’s root folder to ensure that it isn’t doing any redirects or rewrites to the old URL.
Then, using a new browser instance (a private one insures caching/cookies aren’t an issue), I’d try the new URL. If needed, use the browser Developer Tools (usually F12; or right-click and ‘Inspect’) and look at the Network tab to ensure all requests are for the new URL