At this point in time Prisma doesn't support file segmentation. I can recommend 3 solutions though.

Option 1: Prismix

Prismix utilizes models and enums to create relations across files for your Prisma schema via a prismix configuration file.

{
  "mixers": [
    {
        "input": [
            "base.prisma",
            "./modules/auth/auth.prisma", 
            "./modules/posts/posts.prisma",
        ],
        "output": "prisma/schema.prisma"
    }
  ]
}

Placing this inside of a prismix.config.json file which will define how you'd like to merge your Prisma segmentations.

Option 2: Schemix

Schemix Utilizes Typescript configurations to handle schema segmenting.

For example:

// _schema.ts
import { createSchema } from "schemix";

export const PrismaSchema = createSchema({
  datasource: {
    provider: "postgresql",
    url: {
      env: "DATABASE_URL"
    },
  },
  generator: {
    provider: "prisma-client-js",
  },
});

export const UserModel = PrismaSchema.createModel("User");

import "./models/User.model";

PrismaSchema.export("./", "schema");

Inside of User.model

// models/User.model.ts

import { UserModel, PostModel, PostTypeEnum } from "../_schema";

UserModel
  .string("id", { id: true, default: { uuid: true } })
  .int("registrantNumber", { default: { autoincrement: true } })
  .boolean("isBanned", { default: false })
  .relation("posts", PostModel, { list: true })
  .raw('@@map("service_user")');

This will then generate your prisma/schema.prisma containing your full schema. I used only one database as an example (taken from documentation) but you should get the point.

Option 3: Cat -> Generate

Segmenting your schema into chunk part filenames and run:

cat *.part.prisma > schema.prisma
yarn prisma generate

Most of these if not all of them are referenced here in the currently Open issue regarding support for Prisma schema file segmentation https://github.com/prisma/prisma/issues/2377