I'm tecnically an old user (Alice Sacco), but I had to create a new account because i lost the old password and the old E-Mail.
Now i stop to present myself and i ask a question about Sims 4 body types.
I didn't use a lot of mods for sims 4 (unlike Sims 2). In those days I was playing sims 4 and I was looking online solutions to make my sims look better (I also did some easy mods for myself, like T shirts and new hair colours). And I saw some videos of people that have several well-shaped body types and head you can't get with regular sliders. Yet it seems those bodies and heads can be installed all togheter without creating conflict each other or with default settings.
One thing I don't understand is: are those mods made with actual new mesh? Or is more playing with codes? Once i understand how they are made, I want to try myself, to get my characters with the look I imagine for them.

One way without sliders is to use a custom skin. They can have more details and while they don't alter body size, they can change things like body definition like muscles. So a thinner sim can look like a muscular sim or have better body definition than what sliders alone can offer. You apply them to your sims and decide which looks best with your preference just like swapping any other cc. They're usually found in the body details tab. As to making them yourself, that's quite a job, not like recolors or whatnot.

Those presets are made by altering a sim mesh, not with coding.

here's a tutorial on how face presets are made
body presets are made in the same way, but there are less informative tutorials about it, at least from what I could find
https://www.youtube.com/watch?v=vh8I9eTly-g

Can you do a brand full body mesh (and face mesh) like it was done for alternate feminine/masculine bodies?