Artifact can be anything, that you can use clearml SDK to upload to storage. Which storage is used is defined by your clearml.conf (with its credentials) ClearML web and api server do not store those files

Model is a special artifact: None
Example you have the lineage feature where if you train model B using model A as starting point (aka pre-trained) , and model C from model B, ... The lineage will track modelC was built on top of model B, and A.

What is considered as artifact, what as model ? Clearml have some premade code that detect that based on the framework that you use: via the autologging system: None