[hilbert42]

Well, I've looked again and I'm not much further ahead. OpenBoard's help docs are essentially nonexistent, it assumes a standard AOSP Android keyboard so help shouldn't be necessary but there are differences compared with Google's GBoard. Incidentally, my reason for using OpenBoard is that it doesn't spy on one like Google's GBoard.

The only way I can see to get the "care of" character, '℅', in OpenBoard is to use the number toggle and then select the alternate list of characters. As I said this is different to GBoard which allows one the choice between '‰' and '℅' in its long-press of the percent key.

In summary I can't see how I could have entered it from the keyboard without toggling the complete change of characters for the keyboard layout I was using and I don't think I did that—but then I must have. Either that or it's the result of spurious characters arising from hitting multiple keys at once that I mentioned in my last comment.

I hope that helps but I'm still not satisfied with what I've found for the reason that I often want to use Unicode characters that are not on the default keys and I'd to extend that list but it doesn't seem possible. At present, my only solution is to use a separate Unicode app to drop the character in and that's a bit of a pain.