Gtalk is a separate app its not really baked in at all its just preloaded. As for functionality it's closed source so you really can't speculate on how hard it is to add functionality to it. As far as updating it, it would be an update through the play store like every other app so if people allow automatic updates then it could propagate like any other app. Gapps aren't baked in all of them are separate apps that just come with the phone the biggest issue is where OEM's place the app aka /system/app causing you to be unable to remove it.
G+ is a social network and its chat feature is part of that although I would love to see it's hangout feature added into gtalk again both are closed source so we can't.
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Google Talk is XMPP network
it's a open protocol, it's feature specs (aside of core functions which are discribed in main specification) are dividend in to so called XEP expansions, here you have them:
XMPP Extensions ? The XMPP Standards Foundation
Google did some custom once too for gtalk:
https://developers.google.com/talk/jep_extensions/extensions
But still it is fully compatible with XMPP and you can connect to it with any XMPP client and talk to Google Talk users from any other XMPP server as XMPP works like email (again not much people are aware of it as Google Talk is only popular open XMPP network and XMPP community outside google is pretty small and if there any other popular XMPP network most likely people are not aware that they talk between networks)
They use Jingle for Hangouts (as G+ chat is is Google talk) which is part of XMPP:
XEP-0166: Jingle
But it's hard to find any XMPP client that supports it as it's harder thing to implement and don't know if Google implementation is compatible with offical XMPP specs.
Now days most IM (chat) networks are based on XMPP as it open and free and companies can save lot of effort concetrating on making software insted of designing protocol from 0. I think XMPP is reason why lot of services have there own chat system, Facrbook using it, Sony useing it in PSN chat rooms, i even hear Apple used it for there messanger app problem is (as said before) they dont want to open external communication and Google Tlak is only biggest network allowing communicatiosn with other XMPP, but even they have a problem with it as in order to make contact with Google Talk user form other XMPP server that user need gmail address (Google Talk use gmail addess for external communication), otherwise it using some random address and external communication does not seem to work
I recomend you guys to play with XMPP
You will understand how Google Talk really works and what it can do. I belive IM chatting world would be better place if all network used XMPP with external communication allowing to contact anyone on any network, the more people will be aware of XMPP and it's possibilities then better it will be. XMPP (previesly know as Jabber) exists nearly 15 years or more, in now days it's widly used, yet no body is aware of that, mostly because it covered by marketing, as result it does not use it's full potential, like his cousin in network structure... e-mail.