updateOne, updateMany and update operators

updateOne(filter, update) changes the first matching document and updateMany changes all of them. The update uses operators: $set sets fields, $unset removes them, $inc adds to a number, $mul multiplies, $min and $max only change the value if the new one is lower or higher, $rename renames a field and $currentDate stamps the time.

Each operator changes only the named fields, and a change to one document is atomic (all or nothing), even if other clients write at the same moment. That is why $inc on a stock count is safe where read, add and write back is not. The result tells you matchedCount and modifiedCount; check them.