How to describe the implementation of the database in isolation from a specific DBMS?

Question

Database design

It is necessary to tell how I would design a database for a particular business. With a description of all fields, types and relationships. All this needs to be written in a simple text field on the site, so (as far as I understand) no schemes or sample queries are expected from me. Therefore, the question is: how to do this correctly, so that it is clear what is connected with what, and what structure does it have?
As an example, such a simple database


So far, the only thing that came to mind is to write like this

Purchases (
id integer unique not null PK
client integer not null FK (clients.id)
product integer not null FK (products.id)
quantity integer not null
)

Answer 1

Apparently, you are expected to have a logical data model. You can make it in a specialized editor, put the file somewhere and give a link. You can finally give a link to the picture.
But if I were you, I would think about whether it is even worth dealing with those who left only a "simple text field on the site" for such voluminous information.
You can troll them a little more and send an ASCII diagram made with https://asciiflow.com/