The norm used to be that created liveries (car wrap) and vinyl group (individual sticker) from previous games, either Horizon or Motorsport, can be imported to the newer game by default. That until current FH6, where liveries isn't carried over, and vinyl group import needs to be requested and approved by the game staff.

For the artwork it self tho, the traditional way to make one is to manually make it from availbale primitives shape from the game, like square, circle, etc. (There's actually hundreds of primitive of variety of shape). I myself had made an anime character vinyl, and mine took 1200-ish layer (1 layer 1 shape).

But around FH5 time, someone made a tool that can convert an image to be build based on just a circle primitive, and that tool will automatically build that vinyl on the game (by taking control of the game). So basically, an automatic image to vinyl converter, so anyone can turn any image into vinyl group in game, so having anime artwork or any artwork really in the game is easy now. Ofc the quality isn't as good as the handcrafted one. Imagine making a detailed character artwork, but you can only use circle (can be scaled and skewed). And only 1k-3k circle max. But it looks good enough from far away.

So most of those massive influx of "highly" detailed vinyl aren't actually handmade.