Looking at the example with files and folder, can I do something like - give me all the files that are child of a specific folder a user has access to? I don't think it's possible, and I guess it will require to get all the files a user can see and filter them by folder, correct?

Right now you will have to use the list API, but we are already in the process to support such cases better. Please see https://www.ory.sh/docs/keto/guides/list-api-display-objects for how we recommend to solve this rn

Yes, this is what I was thinking - list twice - get all child of a folder and all files I can see and union the lists