On your desktop, this is how the Steam Deck UI will look
The Steam Deck user interface will replace Steam Big Picture mode, but we haven't seen it in action on a big screen yet. Now that we've seen a glimpse of it, it appears to be exactly what you'd expect: the Steam Deck UI on a large screen.
According to screenshots released by Pavel Djundik, the creator of SteamDB, this is the case. SteamOS 3 has leaked, according to Djundik, and some users have begun installing it on other portable PCs (things like the GPD Win 3, or the Aya Neo—basically all existing Steam Deck alternatives). Djundik's photos, on the other hand, were taken in Windows.
As Djundik points out, the UI is still a work-in-progress—after all, it came from a dev kit—so don't mistake these screenshots for the final product. Still, it looks very similar to what we've seen on the Steam Deck, so it's likely to be close to what we'll get in December.
Steam Deck dev kits are being distributed, according to Valve, so expect a lot more leaks in the coming weeks and months. The vast majority of users are unlikely to get their hands on the handheld PC gaming device until well into 2022, due to the reservation situation.