Hello Muhammad,
In all ERP based systems, EHSM needs to be installed on an ERP system as an add-on. Whereas on S/4 HANA EHSM is part of S/4 Core.
Threre are multiple scenarios of how to allow external users access to the system. This is all dependent on the use case.
EHSM does not need a portal in place to access the system, since the user interface is delivered by WebDynpro ABAP utilizing Floorplan Manager (FPM). Only in case you want to allow external users access to the system through the Portal you need to deploy the portal content.
The only component required for deploying EHSM and utilizing it fully is Adobe Document Services (ADS) for report generation. ADS is part of the Java stack, which is in a lot of cases a portal server.
SAP does already bring out of the box e.g. Adobe forms to allow external persons to bring additional data into the system.
A Fiori app deployed via HANA Cloud Platform or alternatively deployed through a demilitarized zone in you own network could allow external persons access.
Kind regards
Joachim