Playback is straight forward. You can choose from a vast variety of players for your pc.
Personal favorite is VLC. Another great player is Media Player Classic. It supports 12-bit playback as well, unlike VLC, MPV and/or SMPlayer
On smartphones, well, latest phones support X265 files playback with the native video player. You may also get VLC for android, or Video Player All Format.
As far as smart TVs are concerned, the latest models include native support for HEVC. Tested on an LG and a SAMSUNG tv.
Mp4 files can be used almost anywhere these days.