How to disable shift + quotation mark vowel diaeresis (äëöïÿ)
What you describe is standard behavior for the "US International PC" input source.  It is intended for Windows users who are familiar with it from that system.  4 keys (` ' " ^) become dead keys for making accented characters used in common European languages.

The fix is to go to system preferences/keyboard/input sources and remove this item and instead use input sources named "US" or "ABC" or "British".

The hardware keyboard called "English International" is the ISO version of Apple's English keyboard. It is used mainly in Europe and has an extra key next to the Z and a differently-shaped return key compared to the ANSI version sold in the US.  It does not require any special input source, all of them work for both ISO and ANSI hardware versions.