What is the primary use of 'will' when making a spontaneous decision?

To express a decision made at the moment of speaking.

Which sentence correctly uses 'going to' to indicate strong evidence of a future event?

Look at those dark clouds! It's going to rain.

The stock market is going to crash soon.

I can differentiate between the uses of 'will' and 'going to' to express future intentions and predictions.

1
We use 'will' for quick decisions, promises, and refusals.
2
'Going to' is typically used for prior plans and when something is about to happen.
3
Both 'will' and 'going to' can be used to talk about future events, but 'will' is often used for decisions made in the moment, while 'going to' is used for pre-existing plans or strong evidence of something happening.