You would be providing an additional specification for each external tool that the AI might find useful to invoke.
There are slightly different specification formats for “functions”, supported on Chat Completions, vs “tools”, also on Chat Completions and the only type of specification that Assistants will accept if you use that instead.
You just have to match up a specification’s properties with the type of inputs your function code needs, and instruct the AI to write the input for it in that special simplified json schema format.
Then either watch the response for those calls to be made, or in assistants poll the status to see if a tool needs to be fulfilled instead of a response to the user being ready.
Your destination for documentation is the “documentation” link on the forum sidebar, then with a section on function calls.