we are planning to use Arcgis Ent 10.9 on Azure . I did not find any article on using APIM with the Arcgis web portal. We intend to segregate the traffic for internal and external users access by hosting two urls.
The architecture flow would be APP Gw--WAF--APIM(External link & Internal link) --LB-WEB adapter .
I think this is going to be a complicated path to go down - though not impossible, it will likely require a lot of trial and error. Using tools like APIM requires a very detailed understanding of how to forward traffic / reverse-proxy traffic through that service. We do have guidance on the Trust Center at https://trust.arcgis.com on Azure WAF Rules, which may help with that tier.
In addition, note that accessing the same Portal/Enterprise deployment using two different URLs is not a supported pattern - the software assumes a consistent WebContext (eg. https://myserver.domain.com/portal) is used by all users who access the Portal. So, I would suggest rethinking that component to identify what is driving you towards separate internal/external access and whether that can be accomplished in a different pattern.