ScyllaDB (written in C++)

ScyllaDB is a distributed wide-column database compatible with the Cassandra API.

Stores data internally using LSM Trees
noSQL databases scale horizontally due to LSM Trees, while SQL(BTree) donot scale horizontally
Super fast, it uses: Seaster Framework
C++ has predictable latency as no Garbage Collector Pauses wrt Cassandra(written in Java).

How ScyllaDB store data?

Scylladb uses Seastar Framework to store data.
Shard (in ScyllaDB terminology): A CPU core inside 1 CPU on single Node(Physical machine), along with the dedicated RAM and network thread tied to that core. See Seastar Framework