These models come from the alpha versions of VRoid Studio and were explicitly released by VRoid under CC0. They were downloaded from here where possible: https://vroid.pixiv.help/hc/en-us/articles/4402614652569-Do-VRoid-Studio-s-sample-models-come-with-conditions-of-use. See that URL for a more detailed preview for now. (AvatarSamples A through C are missing because they were released under different, more restrictive terms. AvatarSamples D through G are unrelated to the samples with the same names in the current version.)

AvatarSample_D_Darkness, the bases, and the hair samples were exported from VRoid Studio 0.14.0. For consistency and flexibility, I've chosen not to use the optimization settings on export; if you want to use this in a game, you may want to merge the textures with whatever tool your software comes with, or download the beta and re-export them yourself.

The files are in the VRM 0.x format, which is an avatar-specific format based on glTF 2.0. This means you can use it immediately in some programs, such as virtual avatar software or certain games.
Look for importers and compatible software here: https://vrm.dev/en/vrm/vrm_applications/. If you can't find an importer for your software, you can change the file extension to .glb and treat it as a glTF file. However, this will remove the toon material, and the Sakurada Fumiriya model in particular may require fixing.