I've put a material (sweater) on a sphere. Now I have a blue sphere but don't see the texture in the viewport?

The sweater material doesn't use textures, it's entirely procedural. If you want to see the results of changes to the material, use the shader view

ok but is it the workflow for all kind of shaders? for example if im using a texture map?

afaik if you specify a texture map, if you copy+paste the address to where the texture lies in the openGL tab of a material inside shops, it should show up in the viewport. Or it did in the days of H8 anyways