Responsibiltiy is shared between AWS and Customer.
There are thing facilities provided to us, and there are facilities we use which sits above facilities provide to us.
E.g. In case of EC2, we are provided everything upto hardware, and from OS to App, we use EC2 for that (Like VM, OS, Storage, App, DB etc.).
E.g. In case of Lamba, we are provided everything upto Storage, we use code and APP on top of that.
What are we provided is AWS's responsibility to make it secure, useful and available to us.
And what we use is our responsibility to make it secure, useful and available to our customer.
And a sum product from this shared responsibility is provided as end solution.