You need to use something like:

from pyspark.sql.types import StructType
 
def flatten(schema, prefix=None):
    fields = []
    for field in schema.fields:
        name = prefix + '.' + field.name if prefix else field.name
        dtype = field.dataType
        if isinstance(dtype, StructType):
            fields += flatten(dtype, prefix=name)
        else:
            fields.append(name)
 
    return fields
 
df.select(flatten(df.schema))