Finding the default isolation level in SQL Server 2017 and Azure SQL DB
It's referring to Azure SQL Database which uses RCSI by default.

Isolation Level SQL
Database default database wide setting is to
enable read committed snapshot isolation (RCSI) by having both the
READ_COMMITTED_SNAPSHOT and ALLOW_SNAPSHOT_ISOLATION database options
set to ON, learn more about isolation levels here. You cannot
change the database default isolation level. However, you can control
the isolation level explicitly on a connection.

In on-premises SQL Server, the default isolation level is Read Committed.
Finding the &quot;default&quot; isolation level is a bit of a goose chase. It will depend on the database where a query is initiated from, and if any hints exist in the query to override a database level setting.
That said, you could query sys.databases to find out if RCSI or SI is enabled.
SELECT d.name, 
       d.is_read_committed_snapshot_on, 
       d.snapshot_isolation_state
FROM sys.databases AS d;

Note that the READ_COMMITTED_SNAPSHOT is not technically an isolation level. Rather, it is a database option that controls the implementation so that row versioning instead of locking is used to provide read consistency for sessions using the READ_COMMITTED isolation level.