What is default Data Blending Join?

The default behaves like a left outer join from the primary data source.

Every row from the primary is kept. Where the secondary has a matching value on the linking field, its aggregated result is brought across; where it does not, the secondary measures come through as null.

Rows that exist only in the secondary do not appear at all. That is why which source you use first in a sheet — the one that becomes primary — changes what you see, and is worth checking when a blend produces fewer rows than expected.

The join type is not configurable in a blend. If you need a different one, you need a real join instead.