what are all the responsibilities or platform team? how they are different from IT/Appain admin team?
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From my perspective these are two names for mostly similar teams. But that depends on the concrete setup.
Yeah I think having more context on what you mean by "platform team" could be really helpful here. One distinguishing factor could be whether you are self-managed or cloud, because sometimes customers will have one team that manages the installation / upgrade while another team manages the Admin Console within Appian. In this case if you are on Cloud the installation / upgrade is all managed by the Appian Cloud team (so the first team wouldn't be needed).
I think there's a lot of possibilities depending on what size teams and applications you have, and every customer approaches it a bit differently!