I am working on a WSGI script that I would like to deploy on my website. The script is my own custom code, and has the standard WSGI interface.
What to I have to do to integrate this into Plone? I have looked at the documents but can't find a good starting point.
Plone itself does not provide WSGI based integration. It is using WSGI to integrate itself into a Web server. This means, the WSGI interface it before Plone (not inside Plone).
By default, Plone 5.2 comes with the "waitress" Web server and uses its WSGI interface. You may try to find out whether "waitress" supports several (so called) applications in the same process and how this is configured.