If it's the latter, an ASRock R1000M is a NUC-sized computer that can drive a 4K monitor for video. It's far more flexible and powerful than any of the tiny proprietary devices. It costs about $250 plus RAM and storage. You can run Plex, MythTV, a full browser, music servers and anything else you might want in a living room.
But if you want cheap and easy, a Roku plus a DNS adblocker works well.
Roku does appear to be the best option indeed, thanks.
What's the best/cheapest cpu/gpu or igpu for tv/movies currently?
I had been using a thinkpad T430 (broken screen and keyboard) as a 'tv-box', but had to switch to a raspberry-pi 4 recently. Sorry to say, the rasp-pi just doesn't cut it. Sure it can output to 2 4k displays, but it can't decode netflix, youtube, or lan (or even sd-card-local) movies smoothly, even at 1080p.
Shield just works.
This gives me youtube videos casting from phone (Newpipe), with API tokens also Twitch.tv. Add an SMB share from NAS/desktop so you can stream your content to the TV over wifi.