>>323166724
Tda (pronounced "tee-da") name of author who made popular hatsune miku 3d model.
MMD: miku miku dance, a popular game/dance animation program that hatsune miku came from.
MMD is heavily modded, so there's tons of ready-made 3d characters for it. Early VRChat had a good workflow so the 3d modelers from that community moved across to vrchat (thank god too, imagine if it was the second life community the influenced vrchat instead). TDA-miku has tons of outfits made, so lots of vrchat avatars are based on that.

MMDC? No idea

>Most of the base models I'd like to use on Deviantart don't have download links or expect me to befriend these weird people and beg them for a link

Yes, welcome to artist communities. They generally want you to jerk them off or provide some other emotional payment for the "free" thing they often stole from another artist.