What? I think you dont understand at all how this works. The database of any password manager is an encrypted file. When you open your password manager and type in the master password it opens that file and decrypts its contents for you and only saves them to memory. It doesnt actually decrypt the file on the drive. When you close the application it doesnt need to be encrypted again. This is exatly the same for all password managers, the only difference is that with web based ones the database file sits on bitwardens server instead of on your harddrive.