Hi HelpfulHare30 ,

1. Is dataset a separate object that can be used within different project or it is a project part?

You can use the dataset in every project you like. Dataset task is part of a specific project, but it can be shared between projects.
dataset_folder = Dataset.get(dataset_id="dataset_id").get_local_copy()

2. Can I find a dataset created with cli in Web UI?

Yes. it should create a Data processing task.

3. Can I configure clearml to store datasets by default on S3?

Yes, you can change it with --storage arg.