Notes on the steps that need more context
Import order and historical orders
Shopify asks you to import products first, then customers, then historical orders, so each order connects to the right product and customer. Products and customers can be imported with CSV files, but historical orders need a migration app or the Order API and Transaction API (Shopify Help Center). Gift cards and store credit move with an app or the GiftCard API.
Products with many options
Each Shopify product can have up to three options and up to 2,048 variants (Shopify Help Center). Products that use more attributes on the old platform need a plan before import: attributes that do not change price or stock can move to metafields, and repeated structured content can move to metaobjects.
Customer passwords
Passwords cannot be moved, because platforms store them as one way hashes. With Shopify new customer accounts, customers enter their email and receive a one time 6 digit code, so no password is needed (Shopify Help Center). Tell customers about the change in your launch email.
Redirects
A Shopify URL redirect only works when the old URL no longer loads a page. Stores can hold up to 100,000 redirects, or 20,000,000 on the Shopify Plus plan. URLs with query strings might not redirect as expected, and redirects do not apply automatically to every market subfolder (Shopify Help Center). Google treats a replatform with new URLs as a site move and advises preparing a URL mapping, redirecting old URLs to new ones and monitoring both. It also says to expect temporary ranking fluctuation while the site is recrawled (Google Search Central).
Private mode and test orders
Private mode lets you keep the store behind a password page while you build (Shopify Help Center). For checkout tests, use the test payment gateway or Shopify Payments test mode. Customers cannot place live orders while payment providers are in test mode, which is why switching it off is a launch day step (Shopify Help Center). Shopify generates your sitemap.xml automatically (Shopify Help Center), so on launch day you only need to submit it.