when video or audio is played from a uri is it streamed or downloaded fully and played?

I have a content creation site I am building and im confused on audio and video.

If I have a content creators audio or video stored in s3 and then I want to display their file will the html video player or audio player stream the media or will it download it fully then play it?

I ask because what if the video or audio is significantly long. like 2 hours for example. I need to know how to solve the use case.

Lastly what file type is most acceptable for viewing on webpages? It seems like MPEG-4 is the best bet. Is that true?