https://learn.microsoft.com/en-us/azure/synapse-analytics/sql/develop-tables-external-tables?tabs=ha...

External tables are not actual tables but a 'view' on top of data which resides somewhere else, NOT in the database.

If you use df.write.format(...sqldw) you are actually gonna write into the database itself which makes it a common table.

If you would use df.write.parquet(<path>) you could create an external table on top of the data, see the link I provided.