phanmetal, I wonder whether you've really answered your own question with your requirement:
It is important that it is Ubuntu or Arch based.
since you will surely meet your other requirement:
a distribution that uses very little ram and runs very few processes (required processes).
by installing a basic or minimal Arch or Ubuntu, and then meet your first mentioned requirement:
by simply installing XFCE on top.
One way of achieving minimal RAM usage, is accomplished by installing a basic or minimal installation and then progressively adding what the user wants, such as X or wayland and then XFCE in your case. In the case of Arch, that's a common approach, but with Ubuntu, it might be more economical to begin with Debian since it underpins Ubuntu, but it may not make much difference there.
The difference in this approach rather than choosing an existing distribution running XFCE, is that the user avoids the overheads and additional programming that the distribution developers have chosen to include in their distro, which they pretty much all do to accomplish particular aims that they have. Their aims however, may not coincide with yours, which in this case is "uses very little Ram". So you have the choice of accepting their versions of a linux system, or making your own depending on how much compromise you are willing to make in relation to RAM usage as you find it.
The main challenge in starting from basics and then adding what you want to the system, is the learning of how to do it, but with packet managers in Arch and Debian, it's greatly assisted.
The least RAM intensive fully-functioning systems I have used are formed from minimal Debian bases and then using suckless utilities like dwm, st and surf. There's no XFCE here, though it can always be installed.
In modern computing, RAM usage has become less of an issue for many users because it's not as expensive as it once was. It's also worth considering that RAM usage in linux is designed to use RAM optimally which means that in many cases it will use as much RAM as there is, if it needs to, but if there wasn't that much of it, its Memory Management processes optimise to use whatever there is so in practical terms it may achieve with less RAM just about what it does with more, but if there is more, it'll use it in accordance with the optimisations built into the kernel. So, essentially, judging systems by the amount of RAM they use can be a fraught matter.