Missing 0 key when change input source from ABC to Hungarian

Hungary (and the rest of Europe) uses the ISO keyboard, which has one extra key compared to the US Qwerty keyboard. As a result, some characters may go missing or be misplaced when you try to use European layouts with the US keyboard.

Hungarian Qwerty has the zero at Option `. Hungarian (which is qwertz) seems to have it at Control Option Zero. (Mac's don't normally use Control for characters like this, so it may not work in all apps.)

You can also create a custom layout, or set up a text shortcut in system preferences > keyboard > text > replace with.

ABC Extended has zero in the normal place and can make all the Hungarian special characters via option dead keys.

https://sites.psu.edu/symbolcodes/mac/codemacext/