you question must be that how axis=0 works because pandas summing across rows means summing up values of each columns.
enter image description here
Now: df.isnull().sum(axis=0)

We’re summing nulls per column, so:

• A → 1 null

• B → 1 null

• C → 1 null

This is summing downward—along rows—affecting each column.

Think of it like:

axis=0:   ↓
          ↓
          ↓