What it finds
Global search looks across every app you can access in the current organization. Each result shows which app it belongs to, and opens the entity itself (not just the list page).
It also filters navigation pages and a few common actions (organization settings, add app).
Pages, actions and templates filter as you type. The other results appear once the search for the current text returns: until then a Searching… row holds their place, and Enter opens the highlighted page, action or template, never a result left over from what you typed before. When they land above the row you moved to with the arrow keys, the highlight stays on that row.
A store that matches shows up once, under Stores. It will not also appear under Transactions.
Ranking
Exact matches on a key, domain, code, or handle rank above partial name matches. That matters when you are debugging a feature gate or atrack() call: type the key you have in code and the catalog entry should land first.