Who should be contacted to design the interaction of all information systems (name of profession, service)?

Question

Preparation of terms of reference

Good day to all!
At the moment, there are the following separate information systems:
1. Mobile application "Events"
2. Mobile application "Personal account"
3. Portal with a personal account
4. Internal corporate portal
4. Electronic document management system
5. Webinar platform
6. 1C software
The plans are to redo everything, to update everything. From the point of view of design, everything seems to be easy: we found a design studio, handed over the necessary information, they, in turn, created a design system where they were competent.
And who to contact for the technical part? Who makes "working documentation" for different information systems so that everything works together (integration), correctly, safely, in accordance with all modern standards? So that they can easily transfer this documentation to different performers, and on the basis of this documentation, they either proceeded directly to work, or already wrote specific technical requirements for each project.

Answer 1

This is what an architect does . There are such varieties: IT-architect, system architect, architect. Just contact someone who knows how to document, and not just write code.

Answer 2

The first step for the IT chief is to document the existing systems. Then you can collect business requirements from departments. Then move on according to the project, updating the documentation.