You'll not be able to set up an iPhone XS from a backup made from an iPhone 8 if it was updated to iOS 12.3.2 because that version of software is not compatible with the iPhone XS software, which if updated is 12.3.1. You can, however, select to restore from a previous icloud backup running iOS 12.3.1 OR you can select to set your device up as new. When setting a phone up as new, as long as you sign in with the same Apple ID and are connected to wifi, all icloud data will migrate to the device.
Here's how to set your phone up as new and get your data on your new phone:
Follow on screen prompts to set up the device. On apps and data screen, select to set up as new.
Once the phone is all the way set up, go to icloud settings and turn on your options such as iCloud Photos, Upload to my Photo Stream, Contacts, Calendars, Notes, and Messages. Messages in iCloud will not default to the 'on' position.
Next, go to settings>passwords and accounts and add your email accounts. Turn on any options for each account such as contacts, notes, calendars, and reminders.
You can change the default for notes, contacts and calendars in settings.
To get all your apps back, go to the App Store>tap 'today' at bottom left>tap the 'photo' in the upper right corner, tap 'purchased' to see a list of all apps that you've previously downloaded with your Apple ID. The majority of apps we use are 3rd party apps, therefore are backed up by their own server. Once you redownload them and sign in with the same login info, they'll be exactly the same.
Please allow up to 24 hours for all iCloud data to migrate to your device.
If you're a member of Apple Music, you'll want to go to settings>music and turn on Show Apple Music, Auto Downloads, and iCloud Music Library. If you're not a member of Apple Music, simply connect your iPhone to a computer with iTunes to sync your music.