Add room into room

Good day,
Hope everybody is doing well and that you enjoyed your weekend.

I like to have the following situation:
I have 3 devices in the kitchen, 5 in the living room and 4 in the dining.
I like to group these rooms into a group or new room, such as ‘downstairs’.
If i say to turn off the lights downstairs, then the lights go off in the kitchen, dining and living at once.

Thank you for answering this nooby question.

Create a new Helper Group (Settings - Devices - Helpers) and add them all into a new Light group. Activating the group will affect all of the entities in the group.