I was curious what information I could glean from these for some popular repos. Caveat: I'm primarily an low-level embedded developer so I don't interface with large open source projects at the source level very often (other than occasionally the linux kernel). I chose some projects at random that I use.
*Mainline linux*
Most changed files: pretty much what I expected for 1 and 2... the "cutting edge" of Linux development over other OSes -- bpf and containers. The bpf verifier and AMD GPU driver might get a boost in this list due to sheer LoCs in those files (26K and 14K respectively). An intel equivalent of amdgpu_dm is #21 in the list (drivers/gpu/drm/i915/display/intel_display.c) and nvidia is nowhere to be seen (presumably due to out-of-tree modules/blobs?).
186 kernel/bpf/verifier.c
174 fs/namespace.c
162 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
161 kernel/sched/ext.c
159 fs/f2fs/f2fs.h
Bus factor: obviously none. The top 4
10399 Christoph Hellwig -> I only know his name because of drama last year regarding rust bindings to DMA subsystem
8481 Mauro Carvalho Chehab -> I also know his name from the classic "Mauro, shut the fuck up!" Linus rant
8413 Takashi Iwai -> Listed as maintainer for sound subsystem, I think he manages ALSA
8072 Al Viro -> His name is all over bunch of filesystem code
Buggy files: Intel comes out on top of GPU drivers this time (twice). Along with KVM for x86(64), the main allocator, and BTRFS.
1477 drivers/gpu/drm/i915/intel_display.c
1406 MAINTAINERS
1390 sound/pci/hda/patch_realtek.c
1102 drivers/gpu/drm/i915/i915_drv.h
943 arch/x86/kvm/x86.c
928 mm/page_alloc.c
871 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
862 drivers/gpu/drm/i915/i915_reg.h
840 fs/btrfs/inode.c
*GCC*
Most changed files: IR autovectorization code, riscv heuristics tables, and C++ template handling (pt.c is "paramaterized types").
152 gcc/tree-vect-stmts.cc
145 gcc/config/riscv/riscv.cc
131 gcc/tree-vect-loop.cc
116 gcc/cp/pt.cc
Buggy files: DWARF debuginfo generation, x86 heuristics tables, RS6000(?!) heuristic tables. I had to look up RS6000, it's an IBM instruction set from the 90s lol. cp-tree.h is an interesting file, it seems be the main C(++) AST datastructures.
1017 gcc/dwarf2out.c
885 gcc/config/i386/i386.c
796 gcc/cp/cp-tree.h
740 gcc/config/rs6000/rs6000.c
720 gcc/cp/pt.c
*xfwm4*
Most changed files: the list is dominated by *.po localizations. I filtered these out. Even after this, I discovered there is very little active development in the last few years. If I extend to 4 years ago, I get:
1. src/client.c - Realizing this project is too "small" to glean much from this. client.c is just the core X client management code. Makes sense.
2. src/placement.c - Other core window management code.
This has not told me much other than where most of the functionality of this project lies.
Bus factor: Pretty huge. Not really an issue in this case due to lack of development I guess.
3298 Olivier Fourdan
530 Anonymous
319 Xfce Bot
121 Jasper Huijsmans
Files with bug commits: Very similar distribution to most changed files. Not enough datapoints in this one to draw any big conclusions.
I think these massive open projects (excl xfwm) are generally pretty consistent code quality across the heavily trodden areas because of the amount of manpower available to refactor the pain points. I've yet to see an example of "god help you if you have to change that file" in e.g. linux, but I have of course seen that situation many times in large proprietary codebases.