Adding a Standalone Software Module

Any kind of interface should be accomplished through a custom developed plugin. If you haven’t already, please review the Plugin Handbook. You will see that at a minimum, there is very little required, yet there are some extremely complex plugins available. It all depends on how much integration your other software requires from WP. It may not require much more than referencing the software output through an iframe, or require a slew of custom functions with action and filter hooks.

It’s critically important to not alter the code of other WP components, especially the core code. WP core offers a plethora of action and filter hooks that allow one to alter what WP does without altering core code. Some themes and plugins also offer many hooks for extensibility, others none at all. If you find there is no recourse but to alter theme or plugin code, it would mean reapplying your changes every time the package is updated, something to avoid if at all possible.

The best solution may not be to alter WP, but to alter the other package. It depends on what needs alteration and how amenable the package is to alteration. From this aspect, WP can be interfaced with through the REST API and/or custom AJAX handlers.