Hi Buck. I'm Greg, an installation specialist and 9 year Windows MVP here to help you.
TwinUI is explained fully here:
https://www.ctrl.blog/entry/what-is-twinui.html
If it's interfering with choosing Default App then see here to Restore the Default File Assocation: https://www.tenforums.com/tutorials/8703-restor...
I hope this helps. Feel free to ask back any questions and keep me posted. If you'll wait to rate whether my post helped you, I will keep working with you until it's resolved.
________________________________________________________
Standard Disclaimer: There are links to non-Microsoft websites. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the sites before you decide to download and install it.