Pre-signed url for multiple files?

No, they would need to provide an API to allow you to upload multiple files first. This is a limitation of the API, not pre-signing.

See Is it possible to perform a batch upload to amazon s3?.

No. A pre-signed URL is valid for only one object.

This is absolutely possible and has been for years. You must use conditions when generating a presigned URL, specifically starts-with. See the official Amazon Documentation.

As an example, here is Python with Boto3 generating a presigned POST url:

    response = s3.generate_presigned_post(
        "BUCKET_NAME",
        "uploads/${filename}",
        Fields=None,
        Conditions=[["starts-with", "$key", "uploads/"]],
        ExpiresIn=(10 * 60),
    )

First things first: AWS S3 is a key value store, each object aaa/bbb/ccc/ddd/index.html is just one name. There is no concept of "folders" ( even though you might have a false impression that they exists from the UI ).

In order to create a single presigned url for multiple "files" you have to do some preprocessing. Pull all necessary files locally, zip them and put zip archive on S3, then generate presigned URL of zip archive.