Jumping into the phone in pocket question.
The simple answer is it depends on the situation and attire.
Girls often put phones in back pocket, because other than purses that usually is the only option, since slimmer/physique hugging clothes and generous side pockets do not mix.(and clothes with large side pockets are less popular in general).
Where I am from guys including myself typically put it in other pockets(shirt, suit, side/front, cargo, holster(older people or trade workers), etc).
However in some instances such as large smartphone or less roomy pants/shorts back pocket may be used. Sometimes if it's needed for "quickdraw" it might go back pocket.
Or in my situation, it sometimes goes back pocket when my front pockets are used for tools, writing utensils, etc during work.
I never have it back pocket when doing anything not on my feet and moving normally. Sitting on it is uncomfortable and could damage it, running or hiking is a recipe for it to have a greater chance of falling out and getting damaged. Additionally depending on environment, back pocket can build up more moisture which is another downside.
No clue about JP culture though, as they have a number of differences. For example to my knowledge it's still somewhat common for them to still use handkerchiefs, while here in the west that has largely been phased out.
Tldr:
Anecdotally where I am from, most guys I personally know typically don't use back pocket. It's not that back pocket is necessarily strange or weird, just seems to be manner of preference that is less common.