Can I use Titanium Backup to extract and restore user apps from a Nandroid backup of a different model ? e.g., restoring user apps from a Galaxy S7's Nandroid backup on a Xiaomi MI Mix 2S?
Yes, you surely can. Make sure that user apps you want to restore are not already present as system apps for example FM radio is a system app with some OEMs while it may be a user app in others. Next thing to watch out is for version incompatibility, though not very common
It takes a long time to restore from Nandroid so be patient. At times if the app doesn't get restored, it's better to install manually from Play Store and restore only data
Also see the last part of answer here Migrating to new rooted phone, how do you restore from your titanium backup from the saved cloud backups