steamos is debian-based

This used to be the case until the launch of the Steam Deck, on which SteamOS (3) is actually based on Arch instead. However, SteamOS is a very special distro based on Arch due to 'immutability', how it achieves said 'immutability', the implications thereof, 'freezing' of packages, inability to install packages persistently without some hacking etc. So, SteamOS is not representative of how Arch works in general.

while popos is ubuntu-based

And Ubuntu is based on Debian.

is that the biggest part of how a distribution works, ie commands, etc.?

If we take your average (popular) distro, so the likes of Gentoo, NixOS etc are dismissed as they are very unique compared to the others, then arguably the most important differentiators would be: Model for updates, package manager and available packages. One might delve deeper into this and with the advent of stuff like Distrobox this becomes a lot more blurred, but traditionally speaking the aforementioned three things used to be the main differentiators. Beyond those, the end-user has the freedom to do whatever with their system. For example, Pop!_OS comes with GNOME + their own touches by default. However, the desktop mode of SteamOS comes with KDE. But you can install KDE on Pop!_OS and even customize it very closely to how it's done over at SteamOS. This is not a special quality of Pop!_OS, but of Linux in general.

Good ui/ux is important for me so i should maybe use nitrux or deepin

It's important to note that both of these are not unique in what they offer in terms of UI/UX. You can recreate 99% of it yourself, simply by installing the appropriate desktop environment; which constitutes most of the UI/UX. Nitrux has KDE as its desktop environment (with a touch of Maui), while deepin uses the Deepin desktop environment. Personally, I wouldn't recommend any desktop environment beyond Cinnamon, GNOME, KDE and Xfce. Don't be discouraged by this though, feel free to put Nitrux and deepin on a Live USB to get a feel for them. Regarding good UI/UX, your best bets are probs Kubuntu, Linux Mint, openSUSE and Pop!_OS. Honourable mention would be MX Linux, but I don't recommend systemd-less distros to newer users.

that are debian-based

Sure, Nitrux is based on Debian. But it's immutable, systemd-less and favors AppImages over Flatpak/Snap. It's a cool project, but I find it hard to recommend to a newer user. While deepin is less unique by comparison, it's far from a distro that's known for its polish. I'd argue it's mostly just eye-candy instead 😅.

or is it a bad idea to choose a less common distro for a amateur like me?

Bullseye! This isn't a hard rule though. I started venturing into Linux through a somewhat obscure distro as well 😅. But, at the time, I researched for about a week which distro to install and why. Afterwards I spent another week on how I should install it and what should be considered for install. And then I installed it, after which I spent almost two weeks getting the system to a working state. It still wasn't quite there yet, but after spending a month on it from start to finish I wanted to move on to something else 😅. I kept the install, don't get me wrong. And it became my daily-driver. After some time I even 'fell in love with it'. But like, I know that I can be stubborn about things like this and persevere where others might have preferred to hit their heads to the wall instead. So your mileage may vary...

Do you have any advice for me?

As you've correctly assessed, you are indeed lost 😅 . That's fine, I think almost all of us have been lost at some point in time. Uhmm..., but honestly, I think you're conflating two very distinct things. Pop!_OS is a general-use distro on which you can do whatever. And most distros that people talk about and engage with are similarly general-use distros. SteamOS, on the other hand, isn't quite like that. Sure, you may hack your way and achieve some things with it. But it's false to believe that you can find any distro that qualifies as SteamOS but on your laptop. Before giving you any recommendations, would you be so kind to answer the following:

• Your post is written in a way that implies that you want to forego Pop!_OS for another distro that's more like SteamOS. Therefore my question would be:
  • What things from SteamOS did you prefer over Pop!_OS? Please be specific and elaborate*.