Where is Resources folder?

I am interested where exactly I should put my materials and textures, that I would be able to use Resources.Load() ? Should I create resources folder inside the Assets folder or somewhere else?

There is no resources folder by default. You have to create it.

Note that Unity considers any folder named resources to be part of the resources folder when building and using Resources.Load() In other words you can have multiple resources folders in a single project. Here are a bunch of valid paths

Assets/Resources
Assets/Resources/MyStuff
Assets/MyStuff/Resources

You could include all or none of them in the same project and access them all with Resources.Load()