MacOS "was" big-endian due to 68k and later PPC cpu's (the PPC Mac's could've been little but Apple picked big for convenience and porting).

Their x86 changeover moved the CPU's to little-endian and Aarch64 continues solidifies that tradition.

Same with Java, there's probably a strong influence from SPARC's and with PPC, 68k and SPARC being relevant back in the 90s it wasn't a bold choice.

But all of this is more or less legacy at this point, I have little reason to believe that the types of code I write will ever end up on a s390 or any other big-endian platform unless something truly revolutionizes the computing landscape since x86, aarch64, risc-v and so on run little now.

To cope with data interchange formats, you need a set of big endian data types, e.g. for each kind of signed or unsigned integer with a size of 16 bits or bigger you must have a big endian variant, e.g. identified with a "_be" suffix.

Most CPUs (including x86-64) have variants of the load and store instructions that reverse the byte order (e.g. MOVBE in x86-64). The remaining CPUs have byte reversal instructions for registers, so a reversed byte order load or store can be simulated by a sequence of 2 instructions.

So the little-endian types and the big-endian data types must be handled identically by a compiler, except that the load and store instructions use different encodings.

The structures used in a data-exchange format must be declared with the correct types and that should take care of everything.

Any decent programming language must provide means for the user to define such data types, when they are not provided by the base language.

The traditional UNIX conversion functions are the wrong way to handle endianness differences. An optimizing compiler must be able to recognize them as special cases in order to be able to optimize them away from the machine code.

A program that is written using only data types with known endianness can be compiled for either little-endian targets or big-endian targets and it will work identically.

All the problems that have ever existed in handling endianness have been caused by programming languages where the endianness of the base data types was left undefined, for fear that recompiling a program for a target of different endianness could result in a slower program.

This fear is obsolete today.