If two people answer, Brainly will put a mark next to the answers that says "Mark as Brainliest". Whichever answer you think is the best, you click the "Mark as Brainliest" and then the answerer gets "Brainliest Answer!" at the top of their answer.
If one person answers and no one else does, Brainly will give you a notification a week or so later, saying something like "Mark the user's answer Brainliest".
Hope I helped :)
