I ask this because i am lost and going crazy since 2-3 days.
Right now on my ubuntu pc i have qt creator and qt designer installed.
I tried to use the qt site help to understand but the explanations are a total mess for me.

Honestly never had luck doing it in that form. I've installed and built the Qt tools on the target, or looked to install the tools on the target via a package installer.

I got Qt installed on Beaglebone Black, Rev C, Debian; via a package and it works fine.

I configured, built, and installed Qt on an Intel ATOM. It took a while and a few iterations to get it working right, but I did eventually get it working. Big ones were to do things like --no-webkit, and no SQL, stuff that I didn't want or need. I tried to cut it back as much as possible, also no demos, not docs, no examples.

Sorry, but never have I gotten it so that I could "build" it on a host and then run it on a target.

Maybe if you have cross compilation tools and build Qt to make static applications it may work, but that might also be very difficult as my configure/build/install path for the ATOM was.

Another suggestion is IMHO that while the designer is helpful, be aware of the code structure you need and be capable of creating your UI from purely code and not a bunch of design patterns. Many Qt examples are purely code, or so I've noticed. I do not use the designer.

The things is that i don't have the choice. It must be a cross compilation from a developement source to a target because of things that i don't control (but i wish i could...) The target i use is sbc6000x with linux 2.6.24 (and busybox 1.19 for some little commands), an old armv5 architecture that i can't update with archlinuxarm or even a debian distrib because of too much limitation or incompatibility and no way to install package on it, only copy paste binaries/folders/symlink will work on it.
This is not a problem, a little learning and it should be good after some mistakes too.