acloud.guru is my #1 choice for pretty much all of the topics you mentioned. Materials there are good enough for any basic and intermediate level of certification.

For advanced/professional level courses I sometimes go for extra content. Examples here:

https://learn.cantrill.io/ for Pro and Speciality - level courses on AWS

https://kodekloud.com/ for k8s

Udemy would have some good materials too, but you need to be careful here... This platform would accept anyone who is willing to produce training videos, so the quality might vary. Look at the reviews and recommendations.

s sc=##class(%ZEN.Auxiliary.jsonSQLProvider).%WriteJSONFromSQL("json","SELECT Name FROM Sample.Person")

While the idea is essentially the same (REST API) - each cloud provider uses slightly different calls and signatures to access data in the storage bucket. So unlikely you'll find a single example that works for every provider.

IRIS External Table  contains Object Script implementation of the storage bucket adapters for AWS, Azure and Google Cloud.

https://github.com/intersystems-community/IRIS-ExternalTable#using-cloud-adapters-outside-of-externaltable - would give you an idea on how to use adapters directly.