The core difference
I'm doing: around now
- Started, not finished
- Now, today, this week, at the moment
- The action is in progress
I do: in general
- A habit or a repeated action
- Always, usually, every day, never
- True in general, not just now
Examples
The water is boiling. Shall I make tea?
Water boils at 100 degrees.
Same verb, different tense, different meaning. The first is about this kettle, this minute. The second was true yesterday and will be true tomorrow.