I once wrote an explanation regarding the ISO v. ANSI keyboards, I copy-paste it here as it seems periodically useful.
The ISO v. ANSI (physical) keyboard
There are two basic types of physical keyboards:
1. the so-called ISO, which has one extra key left to Z (on US iso or other QWERTY keyboards), or left to Y (on QWERTZ and other German-type keyboards). This type of physical keyboard is sold in Europe by Apple and, randomly (and incoherently) by other manufacturers.
2. the so-called ANSI, lacking the extra key; it is the usual physical keyboard sold in U.S. and randomly in Europe. Note that even a good company like Logitech is very unclear when selling their products: the picture on the web page displays an ANSI physical keyboard, but the keyboard inside is, or may be, ISO. For some users, like me, that extra key on the ISO keyboard is very useful.
Type #1, ISO, is very useful in Europe, as many European languages use diacritical marks, therefore getting 4 extra chars is welcome.
What happens if you have a keyboard #2 and use macOS? Look at the pictures: the char left to Z goes on the key left to 1, and what had been there initially… vanishes! Because of this, for years, those who used Romanian or Hungarian could not access all the chars needed. At a given moment, Apple introduced the missing char at the level option+key-left-to-1 for Romanian, but seemingly did not the same for Hungarian, so number 1 could not be accessed. Meanwhile, the problems has been seemingly solved (am not sure, I cannot check now).
So, in principle, in order to avoid this annoying problem, the solution is to repeat (duplicate) the chars available on the key left to 1, which vanish if the physical keyboard lacks the key left to Z. This is what I did with all my custom keylayouts, just in case. I also had 2 macs with U.S. keyboards, so I did for myself, first of all.
Note that UKELELE automatically displays the ISO keylayout if an ISO physical keyboard is attached (my case). Sometimes, it may happen to not identify correctly the physical keyboard, so it should be chosen manually under View/Keyboard type. If you choose ‘3rd party’, it is an ANSI keyboard (the list is long, one may get lost).