What does arch mean in lwIP file names? I tried to guess and think about "archive" or "archaic" but those seem different.
arch in lwIP comes from architecture. As written in documentation of sys_arch.txt in GitHub:
The operating system emulation layer is located in two files, cc.h and sys_arch.c. The operating system emulation layer provides a common interface between the lwIP code and the underlying operating system kernel. The general idea is that porting lwIP to new architectures requires only small changes to a few header files and a new sys_arch implementation.