All your code should be in src folder
You should have index.ts at root level of your src folder
index.ts file should have all exported component, modules, directives, pipes and etc.

Bundle your code

const gulp = require('gulp');
const ngxBundle = require('ngx-bundle');

gulp.task('bundle', async (done) => {
  await Promise.all([ await ngxBundle.rimraf('.tmp'), await ngxBundle.rimraf('dist') ])
    .then(() => ngxBundle.bundle())
    .then(() => ngxBundle.rimraf('.tmp'));
});

Example code

Please see my example code on angular elements on how to use ngx-bundle
Profile Card Angular Elements