You could spend hours on repetitive procedures that could take a lot of time, using a continuous repetitive brush application stroke. Again you could design a particular piece of a pattern or shape you want to use, copy it using a painting program edit menu option of copy and then move your mouse a bit hit the paste command and continue doing this for a repeat pattern. With some painting programs you could create a script to make this repetition easier. When using this procedure once you have a texture created that you find useful, to avoid starting all over again in the future from scratch, select a large area of the texture you have created copy it and then create a blank page in your graphic arts program where you will paste that texture. Save that page in a directory that you accumulate to collect different textures you can use in programs that have layer capabilities, give them titles that you will recognize and make easier to find. In the future you can simply create a layer in something you are working on go to that file you created in the past open it and copy a section of it then paste it into your current image on the layer created for such then use an eraser to remove parts of the texture you do not need.
Another good option is find a program with a wide variety of brushes in it's menu, most programs that cost money have a trial period. Take advantage of the trial program to see what is available in brush variety. You may be interested in buying that program after finding something that has what you are looking for as a continued use. There are a lot of open source free programs that have a variety of texture style brushes such as GIMP, MyPaint, Inkscape (vector application), Krita. Many of these programs have brush editors where you can change the way they apply paint allowing you to possibly create the textures you are looking for, a good commercial program that has this ability is Corel Painter that costs several hundred dollars yet has powerful features for brush creation.
Depends on both the desired texture and the style you want to achieve, but in any case, you need to look carefully at exemples first (real-life or photos), to get a solid grasp of how things work. Notably, in case of textured surfaces, you need to learn how the light interacts with the texture, where the shadows fall, how reflective the material is, and so on. Observation is key in drawing.
Then, after you've understood how the texture works enough to be ready to draw it, you have two options: - If you're using the default round brush, then make this brush very small, lower the opacity (anything between 50 and 70% opacity should be enough), and try to emulate the texture by making tiny dots, scratches, lines, etc. in every areas where the shadows should be, in a darker color than your base color. This gives the most accurate and photorealistic results, but it's time-consuming and some pro artists may look down on such a workflow. - If you have textured brushes, then simply select one with a similar texture (at least from afar, to give the optical illusion that it's the same material) and make random strokes with it on top of the part of your drawing that needs texturing. Depending on the brush you use, you may have to lower the opacity or mess with the settings until you get a convincing result. It's much faster and easier, and the look of textured brushes seem to appeal to a lot of people nowadays.
...Unless you were talking about patterns (like the ones you'd see on fabrics, since you mentioned models), in which case it's another process.