I work for Fermyon, but have been at a major cloud provider in the past, working with cloud for a decade now.
Spin and Fermyon Cloud is a really nice and easy way to get started: https://developer.fermyon.com/cloud/quickstart. It doesn't introduce a whole lot of the inner workings of a cloud. With a lot of other services you'll fairly quickly get to the point, where you'll have to dive in to concepts of distributed systems and infra.
If you want to work with containers and servers in the cloud, fly.io is really easy to get started with as well.
This spec describes how to work with the filesystem in WASI (WebAssembly System Interface). That's the spec adopted by Wasmtime, which is the runtime being used by Spin. (Fermyon employee here)
That's a misunderstanding. Spin runs your webassembly directly from disk. Spin does support distributing application using OCI registries, but they do not run in a container runtime, just a webassembly runtime (Wasmtime).