The memory limit is not (directly) related to uploading of files. It would come into play if you were trying to manipulate the video.
Post max size needs to be larger than upload max file size since uploaded files are encoded and have a bit of overhead increasing their size. I don't recall how much larger post max size needs to be but 10% seems to be stuck in my head.
If post max size and upload max file size are the same you will not be able to upload a file that is upload max file size.
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