When a site loses search traffic after a migration, it’s usually because pages that used to rank simply vanished, leaving old links and search results pointing at an error page. So before the build starts, we list every address your current site has, from its sitemap, Google Search Console and a full crawl, and map each one to its closest equivalent on the new site with a permanent redirect. Pages are never redirected to the homepage as a shortcut, because search engines treat that much like a missing page.
We also carry over the page titles, descriptions and headings that are already working, submit the new sitemap to Google, and check Search Console after launch for anything that hasn’t been picked up. Some movement in the first few weeks is normal while search engines process the change, and nobody can honestly promise exact positions, but a page that ranks today shouldn’t be lost because a redirect was missed.