DTS is a lossy audio format that is the counterpart to Dolby Digital. It had an advantage over Dolby Digital where it allowed higher bitrates. Sony allowed you to choose whether you wanted Dolby Digital or DTS as a bitstream output for game audio on the PS4. DTS is also widely used on DVDs and Blu-rays.
DTS-HD MA is a lossless audio format that is the counterpart to Dolby TrueHD. It's most commonly used on Blu-rays as the audio format.
DTS:X is a 3D object based audio format that is comparable to Dolby Atmos. This is commonly used in UHD Blu-rays, but Atmos has much more support for it at the moment.
ARC allows you to connect devices to your TV through HDMI and then the audio can be passed on to a receiver or soundbar over the HDMI ARC port. ARC was limited to lossy audio only (DTS, Dolby Digital) or 2.0 LPCM.
eARC is part of the HDMI 2.1 standard, although some devices have implemented it without having HDMI 2.1, which is an upgrade over ARC. This allowed for all the lossless formats that ARC didn't allow (Dolby TrueHD, DTS-HD MA, LPCM 7.1).
The problem with the CX is LG stopped paying for the license to process DTS, DTS-HD MA, and DTS:X. This means any device plugged in to your TV cannot pass those audio formats over ARC or eARC. The C9 allowed this but the CX doesn't. A work around is often that a device can decode the DTS track to LPCM. If you set it to do that before sending it to the TV, then the TV will just receive LPCM sound which then will work over eARC. It might be a problem if your audio device only has ARC though since you can't pass 5.1 or 7.1 LPCM through it.
If you pair the TV with a HDMI 2.1 Receiver it would solves the issue correct? Consoles would be connected to the receiver and do the decoding, and TV out to CX.
I'm interested in 77in but kinda wants to see some real world tests when PS5 comes out.