This is code written by others, shipped with the repository so there is nothing to download. Each component keeps its original license, and the license file sits in its own directory.

Directory	What it is	Version and changes	Where it is used	License file
`flip/`	NVIDIA FLIP, an image difference metric that models human viewing. `FLIP.h` plus the source of the command-line tool in `tool/`	Not recorded in the directory	Verification: the image quality measurements in `dm/verify/gfxq/`; build `flip` first with `make -C dm/third_party/flip`	`flip/LICENSE` (BSD 3-Clause)
`libvpx/`	The VP9 decoder from libvpx: the 71 source files a VP9-decoder-only build compiles, their headers, and the generated configuration in `config/`	v1.17.0 (upstream commit `6df3ec3`), unmodified	Host: decodes the games' VP9 movies (`dm/host/hle_movie.c`). `dm/host/Makefile` builds it into `gen/vpx/libvpx.a`; the iOS build compiles the same files	`libvpx/LICENSE` (BSD 3-Clause), with the patent grant in `libvpx/PATENTS`
`stb_vorbis/`	Ogg Vorbis decoder, a single file `stb_vorbis.c`	1.22 (upstream commit `1ee679c`), unmodified	Host: decodes the Vorbis sound tracks of the games' WebM movies (`dm/host/hle_movie.c`, which includes the file with its push-data API only)	`stb_vorbis/LICENSE` (MIT or public domain, your choice; the same text ends `stb_vorbis.c`)
`ryujinx/`	The shader translation library from Ryujinx, three projects: `Ryujinx.Graphics.Shader`, `Ryujinx.Common`, `Spv.Generator`	Ryujinx 1.3.2, with the local changes listed in `ryujinx/README.md`	Referenced by the shader translation tool `dm/gfx/tools/dmsh_xlate`, which translates the original Switch shaders to SPIR-V	`ryujinx/LICENSE.txt` (MIT); `ryujinx/src/Spv.Generator/LICENSE` is that subproject's own license text
`vulkan-headers/`	The Vulkan C headers: `vulkan_core.h`, `vk_platform.h`, `vulkan_vi.h` and the `vk_video/` headers they include	Vulkan-Headers 1.4.363, unmodified	Host: the Vulkan layer on Metal (`dm/gfx/vk/hle_vk.m`) that runs Hades	`vulkan-headers/LICENSE.md` (Apache-2.0 or MIT, your choice)

A few more notes:

flip/tool/ also carries three single-header libraries used by the FLIP command-line tool: stb_image.h (2.23), stb_image_write.h (1.13) and tinyexr.h. Their license notices are in the files themselves.
libvpx/README.md, stb_vorbis/README.md, ryujinx/README.md and vulkan-headers/README.md record where each came from and what it is used for.
Four pieces of third-party code live outside this directory: dm/gfx/nvn/include/nvn.h (MIT, author named in the file header), the layout size algorithm in dm/gfx/nvn/nvn_tex.c, ported from deko3d (zlib license), and two adapted from MacNativeGames (MIT, license text in macnativegames/LICENSE): in dm/gfx/nvn/nvn_mtl.m the GPU widening of 8-bit indices (wide_*) and the in-pass clear triangles (clear_color_tri / clear_ds_tri), from its widen_u8_gpu and clear_quad / clear_ds_quad, and the Tears of the Kingdom visibility-counter lowering in dm/gfx/tools/dmsh_xlate/Program.cs (VertexCounterLowering).