You can connect ADI-2 DAC through USB to your Mac.

The Apollo Twin X doesn’t seem have any digital out, so no connection through that.

I captured these quotes from a RME video. "the dac-fs functions as a normal audio interface" & also "the adi 2 pro fsr is also a very capable class compliant audio interface". I'm a bit confused as on the RME website they a listed under products: Converters. Not under or also under Interfaces... please advise and thank you

For the purpose you asked, best quality monitoring only, the ADI-2 DAC is ideal.
As mentioned, USB works for that, without any add like the BabyFace.

If you have monitors with digital inputs, choose the ADI-2 Pro.
DAC doesn’t have digital outs.
Pro adds digital outs, best quality analog ins and a 2nd set of fully independent analog outputs.

If you instead want ins and outs for typical studio recording work like overdubs, choose one of the recording interfaces, e.g. the UFX II.
The basic difference is the support of “TotalMix FX” that allows low latency direct monitoring, essential for instrument and vocal overdubs.

Numerically the recording interfaces have a few dB lower S/N ratio.
Practically this doesn’t matter, as the figures are already way beyond what any source offers.

If you want to use ADI-2 Pro as recording interface, the combination with e.g. BabyFace, or simply the DigiFace USB, indead is mandatory to get TotalMix FX capability.

To really help you more information is needed.

Most important:

• What exactly do you want to do?
Mixing, mastering, recording, overdubbing, listening to prerecorded music?

• Do you like and use the UAD plugins?
(That’s why one would usually have an UA Appollo Twin X)

• Do you hesitate connecting ADI-2 via USB, if yes, why?
(You’re still seem to try to circumvent USB).

• What is and should be connected, Speakers, headphones, external amps etc. (make and model).

Now we’re on spot.

Simply, yes, ADI-2 DAC connected via USB is clearly the way to go.
I don’t know why you didn’t see a video showing ADI-2 with this connection, but this is the intended standard usage.

You can keep your Apollo or sell it, would only be needed for analog input if you ever need them.
Or as backup, just in case.

I use an Apollo X Twin with my RME AADI-2 Pro FS, e.g.,

1. Interface the AD-2 to your Mac via USB (Stereo or Multi CC).
2. In Mac Audio Midi app, set the ADI-2 as the Output device.
3. However, the monitor speakers are connected to the Apollo X Twin Mon L / Mon R.
4. Using TOS/Link cable, connect the ADI-2 Optical Output to the Apollo X Twin Optical In. You must set up the HW for ADAT 1-2 if running 192kHz (S/Mux), or S/P DIF if running 96kHz or lower.
5. In Your DAW (LUNA) setup an input to receive ADAT ½ or SPDIF.
6. That allows you play or record in your DAW.
7. The Apollon X Twin can run in Stand-alone mode if needed. In Stand-alone, it behaves more like a concummer stereo unit but with 'top=drawer' specs.

Hope this helps.

Billy