For example I don't know anything about shaders, and I wan't to make a glow effect. There are glow shaders made before, in this case copying that is allowed or not?
You can make glow effects VERY easily with Shader Graph if you're using Unity 2019.1 or 2019.2. There are lots of good YouTube videos introducing you to using Shader Graph. You might need to switch to the Low-def render pipeline, which might mean you have to change some existing shades because the default 3D standard shader doesn't work with LDRP.