I have 0 experience in handling dual SIM (which is eSIM + NanoSIM in the case of my iPh12mini) so I'm wondering if there's any dual SIM user here who can share with me your experience! 
Planning to get a 2nd number soon.
Just some of my questions for now:
1) How long can you leave your 2nd number there without feeding it phone credit (e.g. Digi Prepaid) before the number becomes invalid?
Depending on your service provider validity.

2) Can you temporarily deactivate 1 number to save phone battery? If this can be done, can I still receive call on the "temporarily deactivated" number?
You can off either of the lines, when it is off off course cannot answer phone la

3) If I call someone, how does my phone know to use which number to call? Does it ask me which number I want to use each time I make a call?
Thanks in advance if anyone can help! 

I have 0 experience in handling dual SIM (which is eSIM + NanoSIM in the case of my iPh12mini) so I'm wondering if there's any dual SIM user here who can share with me your experience! 
Planning to get a 2nd number soon.
Just some of my questions for now:
1) How long can you leave your 2nd number there without feeding it phone credit (e.g. Digi Prepaid) before the number becomes invalid?

2) Can you temporarily deactivate 1 number to save phone battery? If this can be done, can I still receive call on the "temporarily deactivated" number?

3) If I call someone, how does my phone know to use which number to call? Does it ask me which number I want to use each time I make a call?
Thanks in advance if anyone can help! 

Thanks for the link, very informative!
It doesn't answer the 1st question though.

Depends on the validity. For example, on my work phone I’m using onexox number that has a long validity of 1 year. So i dun need to top up any credit to my Onexox number for 1 year.

If your digi prepaid has 30 days validity period before you need to reload again, then 30 days it is before you gotta feed it credit again.

Sounds like a very good plan the OneXOX. Is it a prepaid or postpaid plan?

However, from the link you shared, if I use lines from 2 different carriers I’ll have to “Unlock” the iphone. Not sure if that’d cause other unforeseen hassles.

Most dual SIM phones has an option to choose your preferred line to make a call out.

Thanks for the link, very informative!
It doesn't answer the 1st question though.