Define each connection in config/database.php, typically using environment variables for its credentials and driver settings. Then select one with DB::connection('connection-name') before running a query or transaction.

An Eloquent model can use a non-default connection by setting its connection property or by calling setConnection for a specific instance or query. Each connection has its own transaction boundary, so a normal database transaction cannot atomically span two unrelated database servers; cross-database workflows need careful design, retries, and compensation where appropriate.