Will this process add to existing bookmarks or replace existing bookmarks in target computer?

Each browser has its own software for this process so different results are possible. In my experience the folder "tree" of the transferred set is copied but may be place under a higher-level folder. No merging of folders occurs, nor removal of duplicates. To unify the organization of your folders after a transfer, most browser support moving bookmarks from folder to folder or copying them to a new folder and deleting them at the original folder. Likewise, commonly you can move entire folders into other folders. To merge two folders into one folder, you normally cut all bookmarks in the folder to be deleted and paste them into the destination. Then delete original folder.