I usually find myself writing Node.js scripts that needs to perform some API calls on GitHub as a GitHub App. I use Octokit.

import fs from 'fs'
import { App } from 'octokit'

const appId = ___
const privateKey = fs.readFileSync(
  process.env.GITHUB_APPLICATION_CREDENTIALS,
  'utf8'
)
const installationId = ___

const app = new App({
  appId,
  privateKey,
})

const octokit = await app.getInstallationOctokit(installationId)