See https://tikv.org/deep-dive/key-value-engine/introduction/. You can specify the storage engine, although all the available engines in that page order the keys: rocksdb allows prefix search, and both B and LSM trees naturally order keys.
In general, a single node partition will use ordered storage. At a higher level (partitions/shards), you use either key ranges or hashing to route keys to partitions: https://tikv.org/deep-dive/scalability/data-sharding/
It seems TiKV uses range partitioning, so that's giving you the order across nodes.