To merge multiple Google accounts, it's possible to consolidate Google Drive and photos into a single account, but merging multiple Gmail, Google Calendar, or Google Contacts accounts is not currently supported. To transfer files from an old Google Drive account to a new one, users need to sync their computer with the Backup and Sync app, disconnect the old account, sign in with the new account, and then select which local folders to backup to Google Drive. For transferring photos from an old Google Photos account to a new one, setting up the new account as a "partner account" allows for automatic copying of all photos without having to download and upload them individually. However, other Google products like YouTube, Google Play, and Google Plus do not have built-in solutions for merging multiple accounts.