Magento with Varnish cache : when is a varnish cache flushed ?

Hello ! I’m currently trying to understand how the varnish flushing is done from Magento.

As I don’t have a lot of insights on the varnish side, I would like to know when the varnish cache flush is done. I’m absolutely not sure that this is correct, but I feel that the varnish cache is flushed everytime a user is logging on the magento backend.. would it be possible ? Does anybody have a list of the operations that can lead to this flush?

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Thanks for now 🙂