A fork can be thought of as equivalent to git clone <URL for source repo>
although the repo on GitHub is bare (no working tree "checked out" files) so behind the scenes they might be doing something like git clone --bare <URL>
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A pull request is asking the owner of the source repo to pull your changes in to their repo ... so there isn't an equivalent git command. It's a bit like emailing the owner of that repo to say "here are some changes I've made, hope you like them". If they accept the pull request they would, basically, then do the equivalent of git pull <URL for your repo>
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