I’m not sure what you mean by “all of halacha was given at Sinai” Obviously this this isnt true.
The halachic process was certainly given but for example many situations hadnt arisen yet. To take a recent example, I dont think anybody would say the halacho of whether or not IVF is allowed was given at sinai (Though I wouldnt be surprised if some people did in fact think so, I have ceased being surprised at some nonsense emanating from certain circles). A framework was given and using that framework well-versed/learned poskim apply it to new situations, thus arriving at different legitimate conclusions.
This is but one example of a source of machlokes but it explains many