ClickHouse Buffers the Query Log and Flushes It Into a Real MergeTree Table
The query log is an in-memory queue drained every 7.5 seconds by a background thread that INSERTs into a lazily created MergeTree table.
System tables
ClickHouse System Tables Are Storages That Generate Their Own Rows
System tables are ordinary storages whose source is rebuilt per query, with a column mask so only requested columns are computed.