While I am on the server edition (which isn't quite up to cloud for UI), one big don't do it thing I note with people is don't put special characters in page titles. I have had to go fix that for people a lot of the time.
That aside, I think what you are looking at there is a listing of Spaces and not pages. As such, it will be displaying the default icon set up for that Space. You can find that under the Configure Sidebar selection of Space Detailes (lower left of the window). Click on the default icon and you can load what you like there.
I'm using Confluence Cloud and I wasn't able to recreate the above page sidebar with Spaces. I am sure they are pages with emojis in the titles. I was about to write a help article on the same thing before I saw this. The above screenshot is from an advertisement video on the official Confluence website:
I just saw that video as well! I agree, I think they're just using emoji in the page title.
While it looks great in the video, I'm not sure how that will affect the pages. The URLs will just be IDs, because you're using a special character. This may of my not be a bad thing.
It's hard to tell if it will affect search results, according to how search is said to work. Buy since these are high level pages, it may not matter as much.
Maybe there are deeper issues I don't know about though. @Mike Rathwell has experience there.
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