when constructing an activty in data transformations, we use insert into, select, where, from. but how we will know we will put some joins in "where"? to sum up, how we can construct an activity?

In a SQL script, a where statement is generally used to filter out data. Joins can also be used as a filtering method to check whether that particular record exists in another table or not or might be used to fetch additional information from the joined table.

This is done as per business need and requirements.