Just wondering. What should i use and for what reason?
A Collection is like a Group/Folder of objects. Collections (among other things) can be made visible/invisible to the render and can be set to only affect the render through indirect lighting, so you can make objects invisible, but still affect the scene. (Collections can also be instanced, used for particle systems, etc.) You can have as many collections as you like and also nest collections within each other.
Layers (View Layers) are configurations of collections. They store which collections are visible, which are invisible, which are masks and which contribute indirect lighting only. When rendering, all view layers that are enabled will be rendered separately for compositing.
Scenes are highest in the hirarchy and store all the data of all the objects, collections and view layers as well as render settings, world shaders, etc. They are similar to a new .blend file, exept they can use the data from other scenes within the .blend file without referencing another file.
You use collections to organize the scene, view layers to render parts of the scene and scenes to basically have 2 .blend files in one. (E.g One scene for rendering and one scene for compositing afterwards.)