How do you add a foreign key to an existing table created by another module?
Create etc/db_schema.xml file with the table node and constraint child node
Run the command bin/magento setup:db-schema:upgrade <title>
This can only be done with raw SQL in a Schema Patch file
Create the etc/db_constraints.xml file and specify foreign key there in the constraint node
https://magento.stackexchange.com/questions/192317/magento-2-how-to-add-foreign-key-in-update-schema
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