Groups are more simple than Layers, and offer a way to bundle objects together for easy selection.
Groups can be created using:
The command group and ungroup
The hot keys command + g for group
The hot keys command + shift + g for ungroup.
The group and ungroup buttons

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💡 Groups work in a hierarchical fashion. Meaning you can have a group within a group. Using ungroup once will release the most recent group. Using it twice will release all groups.
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See extensive documentation of groups here.