How? I’ve installed Debian with KDE
Mistake number 1, Debian is not beginner friendly.
downloaded the .deb from steam website
Mistake number 2, this is windows mentality, if it's not in the package manager it's too advanced for you for the time being. Beginner friendly distros would have had steam in their package manager.
learnt to install that using sudo dpkg -i steam_latest.deb
You could have also double clicked the Deb file, but this is a bad way, dpkg does not resolve dependencies, so you would need to figure those out and install them by hand, which can be tedious at best.
opened the app and i’ve been welcomed with a text inviting me to press enter to continue, pretty simple. The program downloaded stuff, steam is ready now. Not bad.
You lucked out, your system had all of the requirements met.
Repeated the exact same thing on Debian with xfce, that apparently doesn’t come with a software installer, nothing works. An alert says i need to download dependencies (i know dpkg doesn’t resolve dependencies). Where’s the “enter to continue”?
No such luck therez remember when I told you to use the package manager? This is why. Possibly missing something stupid like an i32 library, which you could manually install, but you shouldn't, you're making things hard for yourself for no reason other than wanting to avoid beginner friendly distros.
How is this my fault??
It's your fault because like I've been saying since the beginning you're trying to use Linux as if it were Windows and getting frustrated because it behaves differently. Trying to do this will be frustrating and you will become angry because nothing works like you expect, but you must understand that it's not that things don't work, it's that they work differently.
You might be thinking this is stupid, an installer should install everything it needs, right? Nope, that's a windows mentality, in Linux the main idea is that an installer only installs what it's supposed to, any dependency should be system-wide. Why you might ask? Simple, imagine if every single GUI app had to include it's own copy of the full GUI library it uses, your system would quickly become bloated, not only that but each program would open it's own copy of the library using more and more memory, not to mention the interoperability problems between programs using different versions of the same library. In Linux the standard is for programs to use system libraries, it's the convention, just like how on Windows it is to not (which has its own set of problems). This is why package managers are important, they're not just downloading an executable and running it, they're doing lots of stuff behind the curtains, all of it can be done manually, but like you found out it's troublesome, so best is to avoid.
People downvoting you are very self-centered, they don't understand the importance of a job for lots of people. Until we have UBI, jobs like these will always be necessary.