hdmi passthru vs hdmi earc
If you have a soundbar (or AV, or other audio setup), you have 2 ways to connect it to your TV:
1. HDMI eARC
You connect your input device (like a console or Apple TV ) normally to your TV, and then you connect your soundbar to your TV's eARC port with HDMI cable.
input –(video and audio)→ TV –(audio)→ soundbar
This is in my opinion the easiest and most straightforward way of doing things. Now you just use the TV normally and got audio on the soundbar.
The drawback is that your TV has to support the audio format it's passing through. And if your TV doesn't support a specific audio format then it downgrades the audio to basic stereo audio.
Most high quality 4k movies have audio in either TrueHD or DTS format, which aren't supported by all TVs because a license for them costs. I have LG C2, according to rtings it seems to support TrueHD but not DTS. LG C3 seems to support both.
It might be difficult to find which audio formats your TV supports but try to check rtings.com or flatpanelshd.com.
2. HDMI passthrough
This is when you connect your input device directly to the soundbar (or AV), and let the audio device pass the video signal to the TV.
input –(video and audio)→ soundbar –(video)→ TV
This way you can let the soundbar process the audio however it wants, without it ever touching your TV. This is the best way of making sure you get lossless audio. Assuming your soundbar or AV supports all codecs (they usually do, to the best of my knowledge).
Downside is that you have to make sure your soundbar has HDMI 2.1 to get thet 4k@120 signal to your TV. Many soundbars and AVs has only HDMI 2.0 ports, downgrading the video to 4k@60.
In my experience, another downside is that having another device in your video signal chain adds extra complexity and will cause you some problems sooner or later. At least all the AVs I've used have clunky UI and other problems.
TLDR
If your TV supports TrueHD and DTS audio, use eARC for the most straightforward experience.
If it doesn't, but your soundbar/AV has HDMI 2.1 ports, use HDMI passthrough for lossless audio and video experience.
If your TV doesn't support all audio formats, and your soundbar/AV doesn't have HDMI 2.1 ports, well then you're fucked. Downgrade video (= use hdmi 2.0 passthrough), or accept that some movies will play in stereo (= use hdmi arc, losing support for some audio formats), or buy better gear.
The more I learn about all the different codecs and DRMs and stuff the more I hate it all. Seriously, fuck the whole home theater industry. It's just a time and money sink with extra complexity at every single point.
If I got something wrong in the post, you can email me and I'll correct it :P