Saving Room to Sims 4 Library & Gallery

Save & Share a Room
to library &/or community gallery

In build mode, and in ‘walls-up’ view, click a wall of the room you want to save. This will highlight the room specifically like in the screenshot below:
Click the “Save to My Library” icon…

…from the drop-down select the bottom icon, called “Save Room”.
From the room’s pop-up, name the room, select the room ‘type’, add a description, then select “Save Room to My Library”:
(Alternately you could click the cloud icon and just share to the Gallery at this point, but then you would not have a copy of the room saved to your Library.)
Now your Room has been saved to your Library.
To view your Library items or to share an item from your Library to the Community Gallery, please open the gallery…

… then select the “My Library” tab.
To share to the Community Gallery, select the Room or other item you want to share to the Gallery:
This will open a popup of the Library item. You can edit the title & description if you like here. Now click the cloud icon to share to the Community Gallery. When an item has a green cloud button with a checkmark (or greyed from ‘all’ view), this indicates it is shared to the Gallery. If the item has a blue cloud button with an arrow (or no cloud from ‘all’ view), this indicates it has not been shared to the Gallery: