Some info that's not always apparent when learning how shaders sit in the rendering pipeline :

Shaders can work within the context of the geometry you're rendering, so they're acting upon 3D models for example. Perhaps rendering directly to the screen.

They can also used in a deferred manner, so you render your scene to a texture in the first pass, then you render a quad with that texture applied with pixel shaders acting upon that. it's still the same as the first example, you're rendering geometry (a quad) and shaders are applied, but this setup is intended for full-screen effects such as blur, being applied to what you rendered in the first pass.

Of course it doesn't matter what you texture the quad with, be it a normal texture or a 3D scene rendered to a texture. It's a very flexible setup and leads to much confusion to those learning about shaders. There are many ways to achieve a desired output, some fast, some slower. There's no 'right' way as such. Take this into account as you read tutorials that seem to be setup very differently from each other, whatever works for you / fits your coding style is fine. Don't worry about getting top performance to begin with.