Locks and dirty writes for SQL dbs
Isolation levels are for reading data. Not sure what you mean by "dirty write". Any RDBMS wont allow dirty writes - since they follow ACID properties.

A write operation will always take X (Exclusive locks) till the transaction is completed. Multiple concurrent transactions will use MVCC - sql server uses locking and row versioning since 2005.

Read up - Paul White's series on Isolation levels.