あく = intransitive
あける = transitive
ひらく = intransitive and transitive
Perhaps the thing that will help is to point out situations where only ひらく can be used.
Can you kind of feel what these “opening” actions are like? They involve something kind of expanding
or changing structure so that now the interior can be seen, but you can’t “go through” anything that opened up.
When that kind of motion has happened, you have to use ひらく.
More examples: flowers, wings, websites (getting a bit abstract there, but certainly nothing you can pass through opens)
When some other opening has occurred (such that there is a passage opened, like with a lid or a door) then both are probably acceptable, but あける/あく is probably preferred.
In the intransitive case, you can’t necessarily tell if it’s あく or ひらく visually. You can get a feel for which one is preferred over time, and of course if it’s something like 傘 or 本 then it has to be ひらく.
One thing to note is that for businesses both are possible, but generally speaking 店をひらく means to start a shop, while 店をあける means to open a shop for the day.
If anyone wants to try to tackle it in Japanese, here’s a good explanation.