Just a few examples of it being treated as one letter: if accents are used, it isn't just used on one half of the letter, but on both parts
When someone's name starts with it, its initial will show the whole thing. So for instance IJsbrand Peters will have the initials IJ. P.
In crossword puzzels etc the letter always takes up just one
square. So the word bij takes 2 squares not 3.
And both parts of the letter are capitalized