Main purpose of staging layer in any ETL is to avoid source dependency. Bring the data from the source and keep it in staging tables and then perform necessary data cleansing operations to satisfy business needs. The structure of the staging table idealy same as your source tables, but denormalized in nature according to your data model.
Table 1
product customer invstmt
Table 2
product customer volume
Staging Table
product customer invstmt volume
This the typical example I gave you, decide structure according to your data model the outcome you are expecting out of the package.
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