Can someone help me? Which DAC is being used?

I have a TV with SPDIF out which looks like a 3.5mm jack. I also have an old Samsung 5.1 audio system. It only takes analog RCA input as the only input. If I were to buy an external DAC to connect the TV to the audio system, which DAC is being used? The external DAC or the DAC in the audio system? (PS I’ll only be using it as a 2.1 not a 5.1)

It does not have DAC then.
If it only has RCA (analog signal), the original digital signal has been converted to analog else where.

If the TV actually has S/PDIF, and you connect the new DAC with S/DPIF cable, it will use the new DAC. New DAC then connected with RCA cables to sound system.

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The samsung sound system is actually a home theater system. It plays DVD and CD and does have a DAC built in. But even so, if it takes RCA in only then it wouldn’t be using the DAC. Is that right?

Yes.
it does not have DAC from the view of RCA and that is RCA analog input.

If you burn the audio to a DVD or CD from TV audio’s output and then place the audio disk in audio system. You can use the DAC it has for digital disk’s audio conversion.
It’s possible but not very practical.