Since I got a steam deck, I have been using the app quite a bit, but every time I opened it, it felt like the design hadn't been updated in a long time. The new version of the app that Valve is testing looks dramatically different and is dramatically better than the previous version.
The new design is one of the key new features of the app, and the improvements, at least in my brief testing on the device, are obvious. The navigation bar on the bottom of the screen is where you can find things like the store and news feed.
The app is built on a new framework and has some additional features. What's my favorite? You can use the Steam Guard section to sign into your account, similar to what you can do on the mobile app. You will still be able to do things like browse the store and confirm trades with the new steam app.
The app isn't widely available just yet because it's in the testing phase. If you want to try it out, follow the instructions on the website, but be aware that there are a lot of slots available due to Apple's TestFlight restrictions.