db_schema_whitelist.json file having history of all tables, columns, primary keys, foreign key, constraint keys combination etc, those are created by using the declarative schema.
There are two ways to generate db_schema_whitelist.json by following below methods
Step [1] – Manually To Create db_schema_whitelist.json
Need to create file below path
app/code/VendorName/ModuleName/etc/db_schema_whitelist.json
Step [2] – CLI Command To Create db_schema_whitelist.json
php bin/magento setup:db-declaration:generate-whitelist --module-name=VendorName_ModuleName
It will auto create db_schema_whitelist.json below path
app/code/VendorName/ModuleName/etc/db_schema_whitelist.json