why these Hardware blocks inside an embedded system such as a video decoder?

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Joined Apr 24, 2011
i'm completely newto this domain. I'm reading a book about embedded systems in general and I got blocked regarding a video encoder/decoder chip as an example of an embedded system.
1-Since this chip is an E.S. it needs a processor, RAM and ROM in general. so why a company such as STMicro produce just the processor and the controllers for these RAM & ROMS? why doesn't STMicro or other companies in this field don't leave even the controllers to the RAM and ROM manufacturers?
2-this decoder needs a NIC chip in order to receive signals from DSL modem as an input, so it needs a Network Internet Controller ( which itself is an embedded system with its processor, RAM, ROM,..). Why the NIC is devided into two blocks MAC & PHY and why is just MAC block integrated in Video Decoder Chip and not the whole NIC (meaning PHY block as well)?
3- If some bolocks are in a chip, does it mean that this company has designed and manufactured them? or it's possible for STMicro for example to buy MAC block of NIC from a company and integrate it in its decoder chip? I mean is it physically pheasible?

thanks for any clarification