Using Holdouts in Eevee and Cycles

A Holdout is an object that acts as a mask, creating transparency.

Select the object you want to use and press M to add it to a collection.
Go to the filter icon in the Outliner and make the Holdout icon visible.
Then click on the Holdout icon for the Collection you want to use as a mask.

It doesn’t work until you turn on Transparent under the Film options.

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