Wix lets you add a named custom code snippet and choose exactly where it loads — head or body — and which pages it applies to. For analytics, that means one snippet, set to the head, on all pages.
On most current sites it lives directly in Settings > Custom Code. Some older sites or plans surface an equivalent through the Wix App Market's tracking tools instead — either path lets you set head-level, all-pages loading.
Custom code has been available on both free and paid Wix plans in most current configurations, though availability can vary by site type (Wix Editor vs. Studio vs. Wix Stores) — check your specific dashboard if the option is missing.
Yes, since the "All pages" setting applies sitewide, including product and cart pages, as long as checkout is not redirected to an external hosted domain.
No — Wix's built-in analytics and Orbit's script both read pageviews independently. Running both just gives you two separate counts, not a conflict.