Good morning, Mimyo. Nihonjin desuka.
You don't have to apologise for your language, especially now that you're in Canada. As you saw, even many native speakers don't consciously understand these types of differences.
You've got the basics down,
Basically, I thought,
'Will' -> predicts, plans and etc, but not intended or you are not sure what will happen in the future.
'Be going to' -> same as will, but strongly intended or you have a clear plan to do something,"
but think of the two on a scale, where 'will' represents one end and 'be going to' the other. For some situations, either can be used, this is where they meet in the middle of the scale.
I'll be going to Tokyo on Friday. [decided but the speaker can use 'will', a 'will' of more polite]
I'm going to go to Tokyo on Friday.
I'm going to Tokyo on Friday. [present continuous/progressive for the future]
For some situations, 'will' cannot be used. Imagine a man walking along the street, not paying attention, towards an open manhole.
Seeing this, a native speaker would only use 'be going to', as in
He's going to fall in the hole.
Neither 'will' or the present continuous for the future [PCftF] are available for this situation.
But a general comment about the open manhole with no specific situation present, ie. no man about to fall in, could see the use of 'will'.
Mimyo: [to friend] People will fall in here if they don't put up a guard.
OR a more concerned, serious,
Mimyo: [to friend] People are going to fall in here if they don't put up a guard.
The PCftF can't be used here either, as it's used for relatively near planned events.
Remember that there are lots of situations where either could/can be used, so just because you expect one to be used, a speaker might choose to use the other. Even where it seems 'be going to' is the more natural choice, some will choose 'will' because it is more formal/more polite.
If you have any other questions or if I haven't explained this well enough for you please feel free to ask further.