How does routing work in tRPC especially when using merged routers?

export const apiRouter = router({
  version: publicProcedure.query(() => {
    return { version: '0.42.0' }
  }),
  hello: publicProcedure
    .input(z.object({ username: z.string().nullish() }).nullish())
    .query(({ input, ctx }) => {
      return {
        text: `hello ${input?.username ?? ctx.user?.name ?? 'world'}`,
      }
    }),
})

export const appRouter = router({
  api: apiRouter,
  // ...other routers
})

// server.ts
server.register(fastifyTRPCPlugin, {
  prefix: '/trpc',
  trpcOptions: { router: appRouter, createContext },
})

GET /trpc/api/version
GET /trpc/api/hello?username=Bob

@trpc/server 10.21.0
pnpm 8.2.0
node 18.15.0