Hello again, sorry for bothering xD
I wanted to ask two questions, what are the programs needed to modify the UI of the game? And what are the most recommended tools to start retexturizing objects, skins, etc? (I have some idea because I already made textures for other games, just that I always had the incognita with TS2)
In any case, thanks!

For retexturing, you need Bodyshop for making simple CAS Items (comes with your game). For anything else you need SimPE.

If you want to edit CAS parts, you can do so in SimPE, but you can also use the Wardrobe Wrangler (separate program, allows you to do a lot of edits to the files) and for hairs there's Cat's hair binner (this + their Milkshape plugins here). There are a lot of player-made tools and plugins to make your life easier - you'll find most of those either here on MTS or linked in various tutorials.

You can use any picture-editing program you want, but choose one that can handle layering, picture resizing, transparency, etc., and gives a good quality result (some programs like regular Paint are not adviceable). I prefer Photoshop, but there are cheaper alternatives, such as Gimp (free, though the export for Bodyshop files is a little iffy but workable once you get the hang of it).

If you want to make meshes, you're going to need a meshing program. You can use most of them, but I do recommend Milkshape if you want to work with animated meshes (CAS meshes and animated objects, plus for making poses/animation), but you can also use Blender (free program, there's also plugins for it to assign bones, but from what I gather the export is a little iffy). If you just want to make non-animated objects, you can get away with most programs that can import and export OBJ files (some exported OBJ files may give errors in SimPE if you use the wrong export settings). Personally I use both of them, in addition to Wings3D (for more accurate slot numbers - bit advanced), and I have used 3D Max previously (but that's not cheap...). Blender is a bit difficult to learn, but once you get the hang of it it's pretty good for being a free program.

And that's really all you need. For anything else, make sure to read tutorials on the specific thing you want to do. Extra plugins and programs you need are usually linked in the tutorials, and I think all the player-made ones are free to download and use.

For modifying the UI I think you just need SimPE and a picture editing program. You can download an UI mod and work off that, or extract the files from the game files.

Thanks for the reply!
For picture-editing, same, I use Photoshop.
Currently I do not intend to get into modeling, or touch the meshes, I want to focus more on retexturizing in high quality most of the things(following the visual style of the game, using what is there xD), also had the curiosity to make a more personalized UI.
In any case, really, thanks for the response and for recommending some programs.

You're welcome

A 3D program can actually be quite useful even when you're just texturing. I often use a 3D program for previewing textures I'm working on, and for difficult textures it saves me from going in and out of the game to check adjustments.

I often prefer Milkshape or Wings3D for this, because Blender's way of adding or changing materials/textures is a very roundabout way.