Not a REST call: this is the WebSocket handshake. Connect to the URL below and the exchange answers 101 Switching Protocols; everything after that is frames, not requests.

Public: no key, no signature.

Incremental book updates for the pairs you name. Subscribe exactly as for the trade tape:

{ "Method": "subscribe", "CurrencyPairs": ["BTC_USDT"] }

Apply the deltas to a snapshot from /orderbooks-snapshots or GET /api/orderbook. If you lose frames, re-snapshot rather than patching a book you are no longer sure of.