I have to say, the MMD fanbase is very... 'unoriginal'. For anyone who doesn't know wtf it is, it's a freeware program that was intended for CGI Vocaloid dance videos (Vocaloid is another piece of software entirely). It's incredibly niche, and the majority of people who use it aren't animators by any means - some people are creative enough to make shorts with it, like in SFM, but that's rare and you're more likely to run into the same dance or the same 'meme' animation done hundreds of times by different characters because the original motion data has been passed around like a bad cold.
This also applies to the character models. They have to come from somewhere, and only a few people are skilled enough to make something in Blender, Maya, etc. That's where the old-school salt came from: when the software was younger, the only original models were made by various Japanese artists who put rules on their assets because they didn't want people essentially stealing their content. You'll find countless different models of, say, Miku out there, by different people with different rules. You can download one and check the readme and it might say something like 'free to edit/free to distribute', while another could say 'no edits allowed'.
Nanami was one of the first (2008/9-ish?) to distribute nice looking, high quality models out there, so of course people downloaded that shit like crazy. It would be easier to make their own model by taking parts of another, putting it together and recoloring, which was something Nanami didn't want with his own assets. You can guess people would 'steal' assets regardless of author permission, until eventually he went nuclear over it and stopped anyone outside of Japan from downloading. Cue the western fanbase flogging itself and taking those readmes as gospel in case another model maker did the same.
Since then, that's been way less of an issue with more model makers creating new assets with lax rules, and people stealing anyway because there's nothing really stopping them.
As for YandereDev, if anything, MMD itself is crawling in assets taken from all over the place, I've seen people rip things from H-games, MMOs, other things and rig them to use in this program. You'll see the same hair with different textures on dozens of characters and you'll only know where it's actually from if you're as :autism: as me over polygons.
MMD itself is a really clunky piece of software and you need other add-ons to make anything look competent. There's PMXE/PMDE which are the things people use to edit models with (it honestly looks easier to rig bones and expressions in a real modelling program than this dinosaur.exe), MME (for things like particle effects) and any shaders other than the default are made by and downloaded from other sources. Last time I checked there's no support since 2011 or 2013, even in the years preceding the updates were minimal bug checks.
If you search MMD on Youtube nowadays, it will be crawling with cringy crossovers with FNAF, Undertale, most fandoms you can think of due to the nature of the software, and that's probably the only content (barely) worth sharing here imo.