Transfer Apps from 8+ to XR

Even if the XR is an existing device, it will still wipe the device when you restore the content from the iPhone 8 Plus. That is how a restore from a backup works. It takes everything backed up from the one device (iPhone 8 Plus), and then replaces everything on the target device (iPhone XR). You cannot restore content from one device to another device and mix the two contents together.

Set up your XR from your backup when given that option.

See this, Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Understand that restoring the iPhone XR to the backup of the iPhone 8 Plus, will erase everything currently on the iPhone 8 Plus, and make the iPhone XR look like the 8 Plus, content wise.

The apps are not backed up, your XR will download them from iCloud.

Not necessarily. If you connect the iPhone XR and restore, the restore from backup will wipe it. The only difference is if you are using iCloud to restore, and that requires you to prepare the device differently. Either way, if you want to put the content from the iPhone 8 Plus on the XR, it is going to wipe it.