Regarding 2: that is literally how SMS reactions work. Apps that recognize it just interpret it as "put that emoji on that message". It is unfortunate that it doesn't do that tho.

RCS is different, which you can sometimes get working by installing Google Messages¹, which is essentially the only app that supports RCS any more. Google runs essentially all the servers too.

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1: There are no third-party RCS apps² because, unlike SMS which has an API and a shared database on the device, RCS is extremely locked down and it's literally impossible to create one in stock Android. This is also why it's only "sometimes" on GOS, the details are very complicated and rather enraging.

2: Samsung had one, but they're shutting it down in favor of Google Messages. A tiny number of other devices / telecoms have their own too, but they're rapidly shutting down as well. RCS is very nearly fully controlled and implemented by Google now, except for iMessage as a client only, for now, and there's no encryption between iMessage<->Google Messages last I checked (but there apparently is between Google Messages... but no normal person can really verify that because it's Just Google Everywhere).