Java Card

DF(Dedicated File) ,EF(Elementary File) ,MF(Master File) all of file structure
ISO 7816-4 tell data in card will be in tree structure

SELECT file with get FCI come with response

Now I'm confuse how to keep these type of file to javacard?
Are these file is text file? (with string text in there)
if it's not a text file , What type of file it is? and how to keep data element / record in there

So, I don't understand what is the file ?
And how to make data element keep in file ,how to make tree structure,
how to keep all to javacard

I read specification and understand theory about file & tree structure But don't understand how to make these all in practice.

Can someone make me understand, I will thank you so much

Sorry for my stupid question , but I actually don't know .