See https://marketplace.mendix.com/link/component/117392 for an example.
The PLC data is accessed via APIs.
The PLC integration is part of IIoT solution of Siemens that's comprises of MindSphere, Xcelerator cloud and Data Hub. Most of the assets in the industrial solutions are available as their logical representation (digital twin) via MindSphere APIs. These assets can be a machine or automation systems including PLCs
You should refer to https://siemens.mindsphere.io/en for more information relation to IIoT solutions.