SQL statement sequential execution for multiple statments If a single SQL file contains 2 SQL statements below each other (as below) for millions of records

Query 1 : select Id into MyTempTable from TableB b join abc a on a.Id = t.Id 

Query 2 :  Update TableB set Completed =1

Is there any chance that query 2 will execute first for some of the records or do we have a guarantee of sequential execution of those statements?

In another words, will Query 2 execution be started only after Query1 finishes?
If there are multiple queries in a batch then these queries will be executed in the order they appear in the batch.

After the execution of Query 1 is finished then the update query will be executed.
You can run debugger to see the sequential execution.