Let me 1st say that your idea of "Changing file associations had never been easier" is not shared by me. The old way was much easier and you could change multiple file types at the same time.
Now to the issue. You suggest putting the desired file type in the "Make default association for file type" box, Needless to say there is no such box. In all fairness the format/wording of the box may have changed for no good reason in one of the numerous W11 updates.
However there is a box that does the job as shown above.
Due to your post I revisited the Settings->Apps and was able to make the change for which I thank you.
OOPS - I should have tested it before I wrote the comment. Nothing has changed. I tried again and can't even enter the IrfanView 64-bit entry - I guess I'll have to raise a ticket unless anyone else knows how to change default apps for file types on a vanilla W11 (Oh woe is me to be stuck with W11).