How to extract and enter data on a site without an API, on an aggregator?

Question

API

Site (application) - an aggregator for managing accounts on other sites. These sites do not have api . How does the aggregator interact with these sites? Data extraction is more or less clear (parsers). And what about registration, notifications, and most importantly - data entry through an aggregator?
How to adjust the frequency, withdrawal of money without leaving the aggregator?
ps as an example: there are applications for auto-posting and account management on instagram, vk. If they didn't have an api, how would things work?

Answer 1

The fact that the site has not published the API does not mean that it does not have it. In the end, all data is transmitted in digital form and no "captcha" is asked to be entered - it means that they can be entered without human help.

Answer 2

for insta and vk it all works through the api, it's just not public, you can see the calls yourself in the mobile application or simply by opening the site