All good answers... partly it depends on whether you are trying to analyze an existing data structure (which can be fiendishly difficult if you are trying to disassemble hex code) or start on designing and understanding your own data structures.
The basic, basic idea - if you design your data structures correctly (most C code has lots of DEFine statements or similar at the beginning) then the actual code becomes more straightforward to write.
Object Oriented Programming takes this the concept a little further, in that it usually has tools to adjust and edit "objects" (the data structure) so existing code doesn't need to be modified much if at all... very handy if you've got code you know is good, but the data structure / object format needs modification to make it compatible with some other application.
If you have the (rather old, mid 90s) developer pack, it has examples of typical Amiga structures like IFF (interchange file format) which is probably what most Amiga coders study first. Because, it's used by so many different Amiga applications!
This link is UNTESTED but looks OK from my end, for an ISO of that developer pack... I am confident it will have some examples (in C source code) of typical Amiga data structures.
https://archive.org/details/amiga-developer-cd-v1_1
