Restore a backup from itunes on a PC to an iPhone.

If you don’t know the password, you will not be able to use the backup. You can reset the password for the backup, but that doesn’t help you in this situation.

Are you sure that you aren’t automatically backing up with iCloud? Go to Settings>Your Name>iCloud>Apps using iCloud>iCloud Backup, if it is turned on, you should see the date of the last iCloud backup. You can restore that backup if you have one.

Restore or set up your device from an iCloud backup - Apple Support

Furthermore, you may have been syncing contacts, reminders, calendar events, and Notes with iCloud. If you were syncing those items with iCloud, you can enable them again in the settings under the apps using iCloud section. If you know that you weren’t syncing those things with iCloud, turning them on now will not recover your data.

Alos, apps can be downloaded again in the App Store when you are signed into your Apple ID. Launch the App Store app, tap on your account icon, then tap on Purchased. All of your apps will appear in a list with a download icon. You can download the apps in there. You will not have the data, but you won’t have to pay for the apps again.

Somehow the encrypted backup option got checked. One option that you have is to keep trying various passwords as there is no limit on the number of attempts. Or see the 'If you can’t remember the password for your encrypted backup' section in:

About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

If your backup is encrypted you put a password on your backup, whether you remember doing so or not. That’s the only way encrypted backups get created. Either you checked Encrypt Backup, or, if you have a company email account on your phone, your Exchange administrator required it. But either way you were prompted to enter a password - twice. However, that could have been years ago; you are only prompted the first time you made an encrypted backup, you will never be prompted again until you go to restore a backup.

When you entered the backup password if you have a Mac and have Keychain enabled you were given the option of saving the password in Keychain (this is the default). So check Keychain for it. Unfortunately there is no such option for Windows.

If you haven’t erased the phone yet and it can still turn on you can create a new backup with a new password—>About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

If neither of those work you get unlimited guesses, so try to guess what password you might have entered back when. If you are like most people it will be a password that you reused. And to speed up the process click to change the password and you will be prompted immediately.

Note that if you do NOT encrypt your computer backup then the backup will be incomplete; it will not include health data or passwords. iCloud backups are protected by default because they require your Apple ID and password to access, so this is not an issue for iCloud backups.