Clamshell mode means that you are actively using the MacBook, but on another display. It means that your device is actively working, not sleeping, hence AlDente works perfect as well. There is no more setup to do, I guess.
When your charger is connected and AlDente is set perfect, then when you close the lid, it directly starts the clamshell mode if you have an external display attached to the MacBook. This is what happens on my MacBook right now. I successfully done this and I have not set anything special.
If the charger is connected and if I close the lid, it starts clamshell mode on the second display,
If I unplug the charger during the clamshell mode, it directly goes to sleeping. This is it.
One more thing please check out these settings if they are helpful for you as well.
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