It’s basically a mix of low-poly models and post-processing shaders. I render the game at a low resolution, 640x360 (or you can just use a pixelation shader), then upscale it and add stuff like film grain, dithering, color reduction and a jitter filter. The models are also low-poly with pretty simple textures. Most of them are just noise textures with albedo and normal maps (some also use roughness and metalic).
So basically it’s low-res rendering + low-poly assets + a few shaders layered together.