Install the Redirect contributed module. After enabling it, go to Configuration then URL redirects and click Add redirect.
Enter the source path that should redirect, like /old-page, and the destination, like /new-page. Set the status code to 301 for a permanent redirect, which preserves SEO value from the old URL. Save, and the redirect is active immediately.
For bulk redirects during migrations, use the module’s CSV import feature. Upload a spreadsheet with source and destination columns, and Drupal creates all the redirects in one go. This is invaluable when relaunching a site or changing URL structures. Redirect also integrates with Pathauto, automatically creating redirects when aliases change. This prevents broken links when editors update content titles. Installing Redirect should be one of the first things you do on any Drupal site with significant existing content.