24 hours vs. 144 hours - does border staff in general know the guidelines?

Hi, three related questions:

1. When entering China (Beijing) by flight and leaving less than 24 hours later again by flight - do I have to explicitly choose "24 hours free transit" in contrast to "144 hours free transit" at the border? The difference being the allowance to travel freely within all of China with the "24 hours visa", that is not possible with the other.

2. If for any reason (lack of knowledge or experience at airport border controll) I do not get the 24 hours-visa but the 144 hours visa - are checks that I stay within a certain area around Beijing common? (for example checks on trains or bus-rides)

3. If I go BJ to Mongolia or BJ-North Corea on train and leave China within 24 hours of my arrival: do you think that the Chinese staff at the Mongolian or North Corean border have the knowledge or experience to except my "24 hours free transit visa"?

Thanks in advance!