How can we integrate custom functions into the checkout extensibility feature?

Technical Q&A
1

We need help related to checkout extensibility feature. we going to use checkout extensibility feature but our checkout have some custom functionality then how can we move that in this feature.

  1. is there any API to access or update this checkout feature.
  2. do we have to create app to edit this checkout extensibility feature.
  3. is there any separate API’s for this feature or it is using same checkout API’s.