The iframe's sandbox attribute is doing a lot of work. I tried to change the parent window location to remove the footer, but the sandbox thwarted me since it didn't include "allow-same-origin".
It's collected after 15min, give or take. The hash isn't random, it's the SHA-1 hash of the base64-encoded data, so it's predictable -- if something keeps accessing the base-64 encoded URL, the /s/... URL won't vanish.
I tried making it more strict (by checking the Sec-Fetch-Site and Sec-Fetch-Dest headers), but not all browsers send that.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/if...
Of course, you can just link directly to the URL the iframe points to. In the case of your video, you can simply visit https://smolsite.zip/s/57fe209faf4e6c0316ad32d7eeb792dfb571e... to have it autoplay while getting rid of the footer. I'm not sure how long the /s/ URL stays around before getting garbage collected by the server, but I bet you could regenerate it by sending a GET request to https://smolsite.zip/UEsDBBQAAgAIAPtxJlepozjzcAAAAIgAAAAKAAA... again.