Updated iPhone 6 S plus to iOS 14.5.1 and now it cannot restart past the iniital Apple logo screen.

Hello, Hovawart16.

Thank you for using Apple Support Communities.

From what we read on your post is that your iPhone 6s is stuck at the Apple Logo after updating. We'd be glad to help out. Were these the steps you took to reinstall the iOS again? If you can't update or restore your iPhone, iPad, or iPod touch If not try this first, this will attempt to reinstall iOS without erasing data. Restoring to factory settings should be a last resort because of data loss. Do you know if you have a current backup? If so, then you can restore from a backup after resetting to factory settings.

Also, your MacBook won't recognize the device if it's stuck at the Apple logo. The iPhone needs to fully boot or if you put the iPhone in recovery mode, then the MacBook should detect it. To answer your last question, there is no possible way to downgrade the iOS.

If you're unable to reinstall iOS or restore to factory settings using the link above, then contact Apple Support.

Best regards.