Hi This is a good question. Your questions say "when we use", not something like 'What are the grammar rules for using?' ...and this makes a big difference. You MUST remember 2 things: 1. It is important to learn the grammar rules. I recommend you visit (https://learnenglish.britishcouncil.org/en/english-grammar/verbs/talking-about-future) to do this. According to the grammar rules there are different situations when we use this. Learn the rules. You need them for exams and things like that. 2. Although language learners spend a lot of time learning the rules for this, the truth is that nearly all native speakers (apart from teachers and people who love grammar) either do not really understand the rules for this, or ignore them! We USE all sorts of versions of 'will', 'going to' and present continuous to mean exactly the same thing. So, when speaking to native speakers...forget the rules, and learn how to check if you are unsure exactly what people mean.

Short answer: in practice, they're interchangeable. Most native speakers either don't know the difference or just use the one that "sounds" right. Longer answer: the grammar books will tell you that "will + [verb]" refers to a point in the future (a simple prediction), and "going to + [verb]" refers to a future that is based in, or affected by, what is happening now (for example, a plan or intention). Compare "It's going to rain" (based on the dark clouds I can see right now) with "It will rain" (a simple prediction). Again, this is a distinction that most native speakers don't make, so don't worry about it too much.

when u make quick decision "I will open the door" future facts like telling ur age "I will be 18 next year" on the other hand when u r planning for something in the future. "I have the tickets,I'm going to travel" or something in the future with some evidences "it's cloudy,I think it going to rain" there some other things but I only remember those

Here's a very good explanation: https://www.ecenglish.com/learnenglish/lessons/will-and-going-to