You are mostly correct, the default write policy is ‘update’ which means to merge new bins with existing bins. The exception to this is when using single-bin configuration, in which case all writes are replaces. You can find more information here: FAQ - What is difference between update and replace?.
Notice that even when updating a record that only has a single bin in it, by default (when not using a single-bin configuration), the write will read the existing record to merge the incoming bin with the existing bin, even if they are the same bin.