Streaming Atmos over ARC on LG TVs
Dolby Atmos streams can work over two underlying bitstreams, Dolby Digital Plus and Dolby TrueHD. Dolby Digital Plus is compressed audio, so Atmos with DD+ can be streamed over HDMI ARC. Dolby TrueHD lossless audio has much higher bandwidth requirements, and as a result, requires eARC to support this configuration.
LG OLED TVs did not add eARC compatibility until their 2019 models. This means newer LG TVs can support Atmos with TrueHD, while older models cannot.
Note: All streaming services like Netflix or Apple TV+ support Atmos with DD+ only. Atmos with TrueHD is generally available on Blu-ray media.