There are several different methodes to do a texture mapping in A:M.
(and the great thing about it: You can combine them as you wish)
Many only use stamps in the viewport and just do a little bit of stamping with transparent decals there.
3d painter is no longer under developement, but in general everything works, that can import objs and export textures without changing the toplogy of the polygones to the texture underneath. I really like 3dCoat (very powerful: https://3dcoat.com/) for that.
This is a video tutorial/overview on some of the most common methodes if you really want to do a UV map / unfolding way in A:M.
As mentioned before, it is often not necessary to do a UV mapping in the traditional way but you can do it if you want to.
The tutorial is really just something I've put together in a couple of minutes and there are a lot of things I might not cover in this, but just to give you a small overview). I am trying to show you how you could do a UV map in this, but I am more often going with a 3d painting software like 3dCoat or am just doing everything with stamps itself.
The images show for instance a chamaelon I did with A:M & 3dCoat with color, bump and displacement maps.
And if you'd like to see the stamp way in A:M, I would recommend watching the last part of this tutorial session: https://www.patchwork3d.de/modeling-the-panzer-66-en
If you've got any questions let us know.
Best regards
*Fuchur*
PS: At around 20:40 I am searching for "Apply Snapshot" which is no longer done by hand but can be done form the modelling window/action window/pose window instead and shortens the process of getting a texture with UV tiles on it out a little bit.
PPS: While importing into 3dCoat you can set UV Set Smoothing to "Full UV Set Smoothing" which might help with the seams when you are trying to export displacement maps later on too. If you want to work with this, use 256 (or something very high) when exporting the OBJ from A:M. The closer it matches, the better. But the longer and larger the export / export file will take / be.
video_a_couple_of_ways_to_texture_mapping_in_am.mp4