I need to query for all authors that posted in the last 24 Hours and their latest post that are not older than 24 Hours via the REST Api.
Is this possible? How I'll create a custom endpoint or something?
I am developing an ionic app
You can't do it by default with WordPress, but it is possible with a custom endpoint. You can find the docs in the REST API handbook: https://developer.wordpress.org/rest-api/extending-the-rest-api/adding-custom-endpoints/