Some people use one 24h, other am/pm, but everything related to clocks always has the choice to use one or the other configurable. Most people use 24h format and most public info and tv shows and all that usually uses 24h. When I was a kid am/pm was used in more places, not sure why.

Basically everyone knows how to use both, it's obvious and straightforward, I don't think I've ever seen anyone ever not being able to know the time whichever foat was shown. Am/pm comes more often in informal talking when context of the conversation is obvious since it's much easier to say snack at 3 than snack is at 15.