What is tiflash's storage engine based on?

The official website of TIDB introduces that TiKV’s storage engine is based on RocksDB, but TiFlash is not introduced. What is TiFlash’s storage engine based on?

Just quote from https://docs.pingcap.com/tidb/stable/tiflash-overview

TiFlash provides the columnar storage, with a layer of coprocessors efficiently implemented by ClickHouse. Similar to TiKV, TiFlash also has a Multi-Raft system, which supports replicating and distributing data in the unit of Region (see Data Storage for details).

So it’s based on a modified version of ClickHouse.