There is the following standard NestJS code:

import { Controller, Get } from '@nestjs/common';
import { AppService } from './app.service';

@Controller()
export class AppController {
  constructor(private readonly appService: AppService) {}

  @Get()
  getHello(): string {
    return this.appService.getHello();
  }
}

Also there is the code for module app:

import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';

@Module({
  imports: [],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

I don't understand how app understands that app controller expects to get app service as the first argument? At first I thought that app just looks at providers array and put all services from this array in controller's constructor. But I could change the code in this way:

import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';

@Module({
  imports: [],
  controllers: [AppController],
  providers: [AppService, NewService],
})
export class AppModule {}

Now I can use this NewService not only in AppController class, but in AppService class. How app knows that now I could use NewService in AppService class? Now I don't understand how we could do it, because TypeScript is just syntax sugar and exists only until compilation will be done and as I know app couldn't look at type of argument in AppService (for example fileService: FileService) and put the right service as argument when AppService will be created. I need technical details under the cover of this mechanism, not only use cases from official tutorial.