D, your last question is the key to the answer you are seeking. At WikiTree, a record of a DNA test can be entered on a profile (for privacy concerns, only by the person who took the test or the profile manager of a deceased person). After that, the DNA relationship is traced through the relationship connections that have been made to profiles of that person's family members (but it takes a day after entering the information for it to be traced). If you have added enough family members to find that one already has a profile here, then you can connect with that profile and any DNA matches will trace through that person, as well. You only enter the information about your DNA test - you don't upload the actual data and WikiTree does not do any analysis on it.
You do not want to add a separate profile for a person who already has one because WikiTree is one single tree for (our goal) the entire world. If you have found multiple profiles for the same person, the thing to do is to propose a merge (there is a link to do that at the bottom of every profile page).
Welcome to WikiTree!