OK Basic question on which letters are what in Korean

Still learning the basics. Take this word 예 (Yes) for example:

It’s basically: ㅇ + ㅖ

But at the same time it also looks like:

ㅇ + ㅕ + ㅣ

Looks the same to me (When it’s typed there is more of a space in there). Is there a easy way I can look at a word I do not know and tell if it is ㅖ or ㅕ + ㅣ?

I know it is spelled specifically 1 way and I may just need to memorize each time I come across this type of thing (Which seems often) but looking for a hint or how to better figure it out.

Then I see stuff like ㅙ. Is it typed with two letters ㅗ + ㅐ ? One keyboard has a key for it and others do not as if you have to spell it out. Some have some of the letters when you shift but not all. Pretty much these are the ones I get confused on:

ㅇ+ㅕ+ㅣ is an impossible combination.
One can’t have two vertical vowels in a row.