To set a store as the default, open the store record and select the Default checkbox option, then Save.
Here's the backend behavior for a default store.
Geolocation redirects
The geolocation service only redirects users to a more appropriate regional store if they're currently on the default store (or a store with no path). This prevents redirect loops when a user is already on a non-default store like /eu or /us. See Geolocation.
OAuth/SSO fallback
When an OAuth callback occurs outside a store context, the system falls back to the default to set the current store. See SSO.
Multi-store domain setup
In a typical multi-store setup on a shared domain, one store has default: true with no path, while others have default: false with path prefixes. The default store is typically the root store — the one users land on before being redirected to a regional store.