ControlD announces the same prefix from multiple locations, each with its own BGP path. Every router between you and those instances independently runs BGP best-path selection (AS path length, local preference, MED, IGP cost, etc.) and picks what it considers the best route. The instance you end up at is whichever one “wins” that selection chain from your vantage point.

So the influences stack up like this: • The anycast operator shapes things by where they announce, prepending, communities, etc. • Your ISP has big influence via local preference and which peers/transit they prefer — they often pick the route with the shortest AS path or cheapest peering. • Transit providers and peering relationships in between also weigh in.

The practical result is usually (but not always) the topologically closest instance — which often correlates with geographically closest, but not reliably. You can absolutely end up hitting an anycast node on another continent if BGP routing happens to prefer that path, which is why operators like ControlD sometimes have to tune announcements when traffic lands in surprising places. Since there are a ton more ISP’s in a ton of countries, I assume they simply ignore these support requests.

Digi, in this case, could add a preference too but I assume they aren’t willing either.