@netlify/build/lib/plugins_core/functions_install/index.js

Netlify build module

import { pathExists } from 'path-exists';
import { installFunctionDependencies } from '../../install/functions.js';
// Plugin to package Netlify functions with @netlify/zip-it-and-ship-it
export const onPreBuild = async function ({ constants: { FUNCTIONS_SRC, IS_LOCAL } }) {
    if (!(await pathExists(FUNCTIONS_SRC))) {
        return {};
    }
    await installFunctionDependencies(FUNCTIONS_SRC, IS_LOCAL);
    return {};
};