Under the hood it's webOS, but obviously a custom UI for a TV. I have a 2017 LG with it and it works rather well, no issues with 4K HDR on Netflix and YouTube, and other than booting, it reacts instantly as a TV should. It's a shame there are limited apps for it though - I'd really like apps for Steam Link and Kodi.
I think webOS has been open sourced and there used to be some effort [1] to port it to devices starting with the nexus 4/5. Seems the effort is still decently active with a release just last week[2]
